LED based lighting application

ABSTRACT

The present invention relates to a lighting application, the lighting application comprises an LED assembly comprising a serial connection of two or more LED units, each LED unit comprising one or more LEDs, each LED unit being provided with a controllable switch for substantially short-circuiting the LED unit. The lighting application further comprises a control unit for controlling a drive unit and arranged to receive a signal representing a voltage level of the supply voltage, and control the switches in accordance with the signal. The invention further provides for an LED driver that enables to operate a TRIAC based dimmer at an optimal holding current and an LED driver comprising a switchable buffer, e.g. a capacitor.

FIELD OF THE INVENTION

The invention relates to LED based lighting applications, such aslighting applications that are supplied by a current driver that ispowered by a varying supply voltage such as provided by a TRIAC dimmedelectronic transformer.

BACKGROUND OF THE INVENTION

State of the art efficient and cost effective LED drivers are notdimmable in a retrofit situation secondary to electronic transformerse.g. in the case of being dimmed using a standard TRIAC dimmer. Ingeneral, an LED driver is understood as comprising a power convertersuch as a switching regulator or a linear regulator for powering an LEDor assembly of LEDs and a control unit arranged to control the powerconverter and/or the LED assembly. LED drivers generally are poweredfrom a DC input source where dimming of the light (in response to a userinterface action) is typically realised by adjusting the duty cycle ofthe LED or LEDs of the application. As such, conventional LED driversare not suited for being powered by a voltage source such as provided bya standard TRIAC dimmer. The reason being that the waveform after anelectronic dimmer can vary substantially. As such, the instantaneousvoltage available as input to the LED driver may be momentarilyinsufficient to power the LED or LEDs of the lighting application. Anormal halogen light will average out the power received and will not beinduced to flicker, although even with halogens, the low output levelsare cumbersome and flicker is seen in many cases.

When conventional LED drivers are powered by a voltage source such asprovided by a standard TRIAC dimmer, light flicker may equally occurwhen the current required by the LED driver falls below a minimum valueof the holding current of the TRIAC. A standard TRIAC design may e.g.require a holding current (after firing each 100 Hz cycle in a 50 Hzmains frequency example) of between 30-50 mA. In order to ensure therequired holding current when a voltage needs to be provided to the LEDdriver, it has been proposed in literature to provide a load in parallelto the LED driver in order to ensure that the minimum holding current isbeing supplied by the TRIAC dimmer. Maintaining such a current (in orderfor the TRIAC to maintain its conducting state) may result in animportant dissipation, adversely affecting the efficiency of thelighting application.

Often, state of the art LED drivers to be powered from e.g. a mains ACsupply, apply a comparatively large input filter capacitance (over 1 to10 uF). Such capacitance can e.g. be applied after a unit for EMIfiltering and rectification and before a power converter, e.g. anefficient switching regulator. There are some significant draw backs toa large (over 0.1 uF) capacitance in this location. The capacitor'ssize, weight, cost, reduced life expectancy, and its negative impact onPower Factor Correction (PFC) all lead to serious drawbacks in existingdriver designs when the capacitor is significantly over 0.1 uF.

Another drawback of existing solutions is that significant size, weight,cost, reduced life expectancy of PFC circuitry is prohibitive forapplying low power direct mains current drivers.

A further drawback of existing solutions is that in many cases the 100Hz line frequency is found in the light output. For certain people a 100Hz frequency may easily lead to nausea. In addition, moving the lightingapplication and the eyes of an observer relative to each other may leadto flicker and/or stroboscopic effects at such comparatively lowfrequencies.

It is an object of the invention to at least partially eliminate atleast one of the above-mentioned drawbacks or to at least provide ausable alternative.

It is an object of the present invention to provide i.a. an LED basedlighting application that is better suited for dealing with a powersource differing from a DC power source.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided alighting application comprising

-   -   an LED assembly comprising two or more LED units, each LED unit        comprising one or more LEDs, the LED assembly further comprising        a switch assembly comprising one or more controllable switches        for modifying a topology of the LED assembly,    -   a drive unit for powering the LED units, the drive unit, in use,        being powered from a supply voltage,    -   a control unit for controlling the drive unit, the control unit        comprising an input terminal arranged to receive a signal        representing the supply voltage, and an output terminal for        providing a control signal to the switch assembly to control the        switch assembly in accordance with the signal.

The lighting application according to the first aspect of the inventioncomprises two or more LED units, each LED unit comprising at least oneLED. In accordance with the present invention, an LED is understood asincluding any electroluminescent diode that is capable of generatingradiation in response to an electrical signal. An LED unit according tothe invention may comprise one or more LEDs. In case an LED unitcomprises more than one LED, the LEDs may be connected in series or inparallel or a combination thereof. The LED units as applied in thelighting application according to the invention are arranged in a serialconnection.

The lighting application according to the first aspect of the inventionfurther comprises a switch assembly for modifying a topology of the LEDassembly. As an example of such a modification, short circuiting one ormore of the LED units can be mentioned. The switch assembly can e.g.provide each LED unit with a controllable switch for substantiallyshort-circuiting the LED unit. The switch assembly may, as an example,comprise a FET or MOSFET connected in parallel with one or more of theLED units. By controlling a state of the FET or MOSFET, (i.e. conductingor non-conducting), the one or more LED units may be short-circuited. Inaccordance with the invention, modifying the topology of the LEDassembly is understood to include, but not being limited to:

-   -   connecting or disconnecting one or more LED units such that they        are no longer powered by the drive unit;    -   short-circuiting one or more LED units such that a supply        current from the drive unit flows through a parallel path to the        LED unit and no longer through the LED unit itself;    -   modifying how the LED units are interconnected.

As an example of the latter, changing a series connection of two LEDunits of the LED assembly to a parallel connection of the LED units canbe mentioned.

The lighting application according to the first aspect of the inventionfurther comprises a drive unit for providing power to the LED units.Within the meaning of the present invention, a drive unit is alsoreferred to as a converter or power converter. As an example, the driveunit may comprise a buck converter or a boost converter. Such aconverter can convert an input power source (e.g. a supply voltage froma dimmer circuit) to an appropriate current source for powering one ormore LED unit. Converters such as a Buck or Boost converter are examplesof switching regulators. It is worth noting that the drive units asapplied in the lighting applications or LED drivers according to thevarious aspects of the present invention can also be linear regulatorssuch as voltage or current regulators. The use of such linear regulatorsis often discouraged because of a poor efficiency in case of animportant mismatch between the supply voltage and the required loadvoltage. In accordance with an aspect of the present invention, thetopology of an LED assembly can be modified based on a signalrepresenting the supply voltage. By doing so, a better match between thesupply voltage and the required load voltage can be realised, as will beillustrated in more detail below. As such, in a lighting applicationaccording to the present invention, a linear regulator can be applied ina more efficient way. As an example, a rectified AC mains voltage (e.g.230 V, 50 Hz) can be used as a supply voltage for a linear regulatorpowering an LED assembly. Assuming each LED unit of the LED assembly tohave a forward voltage of approx. 4 V and a minimal voltage drop of theregulator being 1.5 V, the control unit controlling the switch assemblycan modify the LED assembly topology by adding a series connected LEDunit to the LED assembly when the voltage difference between the supplyvoltage and the load voltage (i.e. the voltage over the LED assembly)exceeds 5.5 V. When the supply voltage reduces and the voltagedifference approaches 1.5 V, one of the LED units can e.g. beshort-circuited, e.g. by closing a switch of the switch assembly by thecontrol unit. As an alternative, the topology can be changed byconnecting LED units in parallel rather than in series.When and how to change the LED assembly topology can be determined bythe control unit based on a signal representing the supply voltage, ingeneral representing a property of the supply voltage. The signal cane.g. represent the difference between the supply voltage and a requiredload voltage, e.g. the required forward voltage over the LED assembly.In an embodiment, the signal can, as an alternative, represent the loadcurrent as provided by the power converter to the LED assembly.As a result of the described way of controlling the LED assemblytopology (and thus the required load voltage) by appropriate switchingof the switch assembly, the losses in the regulator will approx. varybetween 1.5 times the applied current and 5.5 times the applied current.

In accordance with the first aspect of the invention, the lightingapplication further comprises a control unit for controlling the switchassembly. The control unit can be implemented in various ways such aswith dedicated hardware, using one or more microprocessors or digitalcontrollers that are programmed using software. The control unit asapplied in the present invention may also be implemented as an FPGA oran FPGA with a soft-core processor or as an analogue or digitalcontroller.

The control unit as applied in the lighting application according to theinvention is arranged to control the switch assembly based on a signalrepresenting the supply voltage (or one or more characteristics, e.g. avoltage level, of the supply voltage) as applied to the drive unit (i.e.the power converter (e.g. a Buck converter) of the lightingapplication). As an example, the control unit may determine a maximumnumber of LED units or which LED units that can be powered (e.g. basedon the available supply voltage and information regarding the requiredforward voltage of the different LED units) and control the switchassembly in accordance. The signal representing the supply voltage (orone or more characteristics of the supply voltage) can be deriveddirectly from the supply voltage provided to the drive unit or may bederived from a different access point on the lighting application. Thelighting application according to the invention can e.g. be powered froma mains AC power supply. In such case, the AC power supply may undergovarious transformations prior to being used as a supply voltage to thedrive unit. Such transformations may e.g. include an actualtransformation to a different voltage level, a rectification by arectifier, filtering, reduction via a dimmer circuit such as an externalTRIAC dimmer circuit. In between such transformations, the voltage canbe accessed and can be used to derive a signal representing the supplyvoltage of the drive unit from.

The lighting application according to the first aspect of the inventionthus facilitates the application of a varying voltage source as a supplyvoltage for a drive unit for powering two or more LED units. In case thesupply voltage as provided to the drive unit drops below a requiredvalue for powering all LED units when connected in series (i.e. therequired forward voltage over the serial connected LED units), thecontrol unit can e.g. determine the maximum number of LED units that canbe powered at the same time and control the switch assembly to shortcircuit one or more LED units to secure that the required forwardvoltage can be provided by the supply voltage. Instead of shortcircuiting one or more LED units, changing the interconnecting of theLED units from a series connection to a parallel connection can beconsidered as well. In order to comply with a requirement of providing acertain average light intensity, a momentarily lower intensity, due to areduced supply voltage, can be compensated later by providing anincreased intensity.

As such, the lighting application according to the first aspect of theinvention is particularly suited for retrofit applications. At present,different types of lighting applications are applied in e.g. domesticenvironments. Such applications e.g. include light bulbs which can e.g.be supplied directly from an AC mains power supply (e.g. 230V, 50 Hz) ora dimmed AC supply (e.g. an output voltage from a TRIAC dimmer). Otherknown lighting applications are e.g. supplied from a comparatively lowAC voltage (e.g. 12 V or 24 V) which can equally be dimmed. Knownlighting applications also include light sources powered by a DC voltageor a DC voltage having an AC component superimposed to it. The lightingapplication according to the present invention can be powered from avariety of power sources as the lighting application is arranged toadjust the effective number of serially connected LED units (and thusthe required forward voltage of the serially connected LED units) basedon the momentarily amplitude of the power supply voltage that isavailable. The lighting application according to the present inventioncan thus be supplied from an output voltage of a dimmer circuit such asa TRIAC dimmer.

In an embodiment, the control unit as applied in the lightingapplication is arranged to determine a dimming level from the supplyvoltage. As explained in more detail below, this can be accomplished indifferent ways; The required light intensity (or dimming level) can e.g.be determined from an average value of the dimmer output signal. Thelighting application can e.g. be arranged to assess such average valueand provide a signal representing such average value to the controlunit. The control unit may, in response to the signal, control theswitch assembly to obtain the required light intensity.

As an alternative, the dimming level can be determined or estimated froma duty cycle at which a switching element of the drive unit or powerconverter (assuming a switching regulator is applied) is operating. Aswill be explained below, the duty cycle of such a switching element canvary depending on the difference between the available supply voltage(e.g. a dimmer output voltage) and the required load voltage. As such,the observed duty cycle or a signal representing this duty cycle mayequally be applied to control the topology of the LED assembly.

In an embodiment, the lighting application according to the inventionfurther comprises a rectifier for rectifying the dimmer output voltageand outputting the rectified voltage as a supply voltage for the driveunit.

In an embodiment, the lighting application comprises a waveform analyserarranged to assess the supply voltage and/or any internal voltage andprovide the signal (i.e. the signal representing a voltage level of thesupply voltage) to the control unit. Such a waveform analyser can e.g.comprise an A/D converter for converting a signal representing thesupply voltage. The waveform analyser may e.g. comprise one or morecomparators for determining a voltage level of the supply voltage. Thewaveform analyser can e.g. be arranged to determine a zero crossing ofthe supply voltage. As such, the waveform analyser may facilitate asynchronisation between a periodic supply voltage (e.g. a (rectified) ACvoltage or TRIAC dimmer output voltage) and the control signals of thecontrol unit as applied in an embodiment of the lighting applicationaccording to the first aspect of the invention,

Note that a one-to-one correspondence between the supply voltage (inputfor the drive unit) and the required forward voltage may require somescaling: relevant for the determination of the (number of) LED unitsthat can be powered is the output voltage that can be generated by thedrive unit given a momentary supply voltage. This output voltage cane.g. be somewhat smaller than the supply voltage (e.g. due to voltagedrops inside the drive unit, . . . ).

When supplied from an AC power source, the input AC wave form (or thewave form that is outputted by an (electronic) transformer such as adimmer) can, in an embodiment of the present invention, be used by theLED driver to synchronise a number of control and feedback methods (e.g.by the control unit) that allow for a cost effective, power efficient,dim-able, and retrofit-able LED driver.

As known to the skilled person, in order to change an intensity of anLED or change the colour of the light generated by an LED assembly, theduty cycle at which the LED is operated can be altered. As an example,when an LED is provided with a current during 25% of the time (i.e.operating at a 25% duty cycle), the intensity of the light is reduced tosubstantially 25%. In practice, the on and off cycling of the current(resulting in either generating light or not) can be performed at asufficiently high frequency such that it becomes unnoticed by a person.In order to achieve this, in general, a duty cycle period ispredetermined and the required duty cycle is applied within said period.Within the meaning of the present invention, the term ‘duty cycleperiod’ is used to denote the period over which a required duty cycle isapplied. As an example, a duty cycle of 25% can be realised in apredetermined duty cycle period of e.g. 4 ms by providing a currentduring 1 ms and subsequently turning off the current during 3 ms andrepeating this process. By selecting the duty cycle period sufficientlysmall and thus having a sufficiently high frequency content for thecurrent, no intensity variations will be observed by the human eye. Inthis respect, it is worth noting that the ON and off times of the dutycycle need not be continuous within the duty cycle period as meant here.See, for example, WO 2006/107199 for a more elaborate build-up of the ONtime within the duty cycle period. In general, the applied duty cycleperiod is predetermined and e.g. based on the possible (minimal) pulsewidth of the current that can be provided and/or the resolutionrequired. Assuming a current pulse having a duration of 2 microsecond ora multiple thereof can be generated, the selection of a duty cycleperiod of 4 ms would then allow the intensity to be varied in 2000 stepsbetween full intensity and zero intensity. In case an LED assembly ispowered from a substantially constant power source (e.g. a DC powersupply), the selection of duty cycle period can be done arbitrarily andvarying the duty cycle period slightly would not affect the illuminationas observed.

However, when an LED assembly is powered from a periodic supply voltage,it has been observed that a particular selection of the applied dutycycle period with respect to the period of the supply voltage canprovide certain advantages. In order to avoid aliasing effects due tothe interaction of the frequency content of the supply voltage and theduty cycle modulation as e.g. applied by the control unit to the LEDassembly, it has been found that a duty cycle period should be selectedsuch that the division of the supply voltage period by the duty cycleperiod results in an integer number.

As such, according to an embodiment, the control unit is arranged toapply a duty cycle modulation to the LED assembly by controlling theswitch assembly, whereby the duty cycle modulation is applied with aduty cycle period selected such that a division of a supply voltageperiod by the duty cycle period results in an integer number.

As an example, for 50 Hz applications (the period of the rectifiedvoltage being 10 ms), the duty cycle period can be selected to be 5 ms,for 60 Hz applications (the period of the rectified voltage being 8.333ms), the duty cycle period can be selected to be 4.165 ms. In a yetpreferred embodiment, the duty cycle period is selected such that thedivision of a first supply voltage period by the duty cycle periodresults in an integer number and the division of a second, differentsupply voltage period by the duty cycle period also results in aninteger number. By doing so, the lighting application can be poweredfrom power supplies with a different frequency while maintaining theadvantages of avoiding aliasing effects. As an example, selecting theduty cycle period equal to 833 microseconds results in both the 50 and60 Hz period being so divisible by the duty cycle period. By selecting aduty cycle period as indicated, the switching of the switch assemblyenabling the duty cycle modulation becomes in fact synchronised with thesupply voltage.

In accordance with the invention, the switch assembly (i.e. one or moreswitches such as MOSFETs, FETs, GTOs, IGBTs or the like that arecontrolled by a control unit) can serve two purposes:

-   -   the switches can either be applied to modify the topology of an        LED assembly as described above, and/or,    -   the switches can enable the LED units of the LED assembly to        operate at a desired duty cycle thus obtaining a duty cycle        modulation of the current through the LED units, e.g. PWM or the        like.

As explained in more detail below, the lighting application according tothe first aspect of the invention may further comprise additionalmodules such as an EMI filtering module, a Power Factor Correction(PFC), an input filter to the LED driver optionally including a(switchable) buffer and an output filter optionally including a(switchable) buffer arranged (i.e. electrically connected) between theLED driver and the LED assembly of the lighting application.

The first aspect of the invention can also be implemented without theuse of a specific drive unit such as switching regulator or a linearregulator. As such, according to the first aspect of the invention,there is provided a lighting application comprising

-   -   an LED assembly comprising two or more LED units, each LED unit        comprising one or more LEDs, the LED assembly further comprising        a switch assembly comprising one or more controllable switches        for modifying a topology of the LED assembly, the LED assembly,        in use, being powered from a supply voltage,    -   a control unit for controlling the switch assembly, the control        unit comprising an input terminal arranged to receive a signal        representing the supply voltage or a load current to the LED        assembly, and an output terminal for providing a control signal        to the switch assembly to control the switch assembly in        accordance with the signal, thereby modifying the topology of        the LED assembly.        In such an embodiment, the LED assembly can directly be coupled        to the supply voltage, whereby the topology of the LED assembly        can be adjusted or modified depending on the available voltage.        By changing the way the LED units are interconnected (e.g.        either in series or in parallel), the required load voltage        (i.e. the forward voltage required by the LED unit or units) can        be altered and adjusted in accordance with the available supply        voltage.

In an embodiment of such a lighting application, the control of theswitch assembly is based on a signal representing the supply voltage.Such a signal can e.g. be a digital signal proportional to theinstantaneous amplitude of the supply voltage. As an alternative, thesignal could e.g. indicate when a certain voltage level occurs. Thesignal could thus e.g. be a pulsed signal comprising a pulse when thesupply voltage equals zero. Such a signal can e.g. be applied in casethe supply voltage is a periodic voltage with a fixed frequency andamplitude. As such, the notion when a certain voltage level occurs (e.g.a zero-crossing) can be sufficient for a control unit controlling theswitch assembly to determine how to modify the topology. As an example,in case the supply voltage is a rectified 230V, 50 Hz AC mains voltage,the control unit can determine at any instance the available voltagewhen e.g. the instance of zero voltage is known. Based on the availablevoltage, the control unit can control the load (i.e. the LED assembly)such that the required load voltage substantially matches the supplyvoltage.

It is worth noting that, when relying on a voltage measurement in orderto determine when and how the LED assembly topology should be modified,measures are preferably taken to limit or control the current asprovided to the LED assembly. As will be understood by the skilledperson, the current vs. forward voltage characteristic of an LED is verysteep beyond a certain forward voltage over the LED. In order to avoidexcessive currents which could damage the LED, a current limiter orcurrent limiting measures as known in the art can be implemented in thelighting application.

In another embodiment of such a lighting application, the control unitcontrols the switch assembly based on a signal representing a loadcurrent to the LED assembly. As will be understood by the skilledperson, when an LED is provided with a voltage above it's nominalforward voltage (e.g. 4 V), an important increase of the current throughthe LED can be observed. Equally, when an LED is provided with a voltagebelow it's nominal forward voltage (e.g. 4 V), an important decrease ofthe current through the LED can be observed. As such, the current asprovided to the LED assembly, provides a clear indication of therelationship between the available voltage and the required loadvoltage. As such, the control unit can control the switch assemblyaccordingly and thus change the LED assembly topology. As an example,the control unit can control the switch assembly to add a seriesconnected LED unit to the LED assembly when the load current increasesabove a certain level and to short-circuit an LED unit (or modify theinterconnection of an LED unit from being series connected to parallelconnected) of the LED assembly when the load current drops below acertain level (both levels can either be different but could also be thesame).

Controlling the switch assembly based on a signal representing the loadcurrent to the LED assembly can be advantageous as it provides a moredirect approach to determining whether or not to add or remove a LEDunit, in general, modify the LED assembly topology, compared to e.g.assessing a difference between the supply voltage and the load voltageand control the switch assembly using this difference.

The load current as applied to control the switch assembly (via thesignal representing the load current) can either be the total currentprovided to the LED assembly or the current provided to one or more ofthe LED units. In case the total load current is used to control theswitch assembly, the actual topology (in case two or more LED units areoperating in parallel) of the LED assembly may need to be taken intoaccount. As such, the level (or levels) used to trigger a change intopology may need to take the number of parallel branches of the LEDassembly into account to assess whether the current supplied to the LEDunits is above or below a certain level.

As adding or removing LED units to or from the LED assembly (due to achange in topology of the LED assembly) can affect the load currentprovided to the LED assembly and can thus cause current variations, thepresent invention proposes different measures to mitigate the currentvariations.

As an example, the duty cycle at which the LED units are operated can beadjusted such that brightness variations due to load current variationsare mitigated. As an example, the duty cycle can be gradually reduced incase the load current increases above the nominal current therebymaintaining the brightness substantially constant. Such an adjustment ofthe duty cycle can e.g. be realised by appropriate control of the switchassembly or, when applied, the drive unit for powering the LED units.

As a further example, current variations can be mitigated by applyingthe voltage difference to charge a capacitor prior to adding a seriesconnected LED unit (when the supply voltage increases) and dischargingthe capacitor to maintain the voltage over the LED assembly when thesupply voltage decreases.

The first aspect of the invention further provides a method of poweringan LED assembly by an LED driver connectable to a power source, the LEDassembly comprising a serial connection of two or more LED units, eachLED unit being provided with a controllable switch for short-circuitingthe LED unit, the method comprising the steps of

-   -   detecting a voltage output level of the power source,    -   compare the voltage output level to a required voltage for        powering the LED units to determine a maximum number of LED        units that can be powered by the power source,    -   control the switches of the LED units in accordance with the        maximum.        The method for powering an LED assembly according to the first        aspect of the invention enables to power an LED assembly taking        into account the available supply voltage of a power source. As        explained in more detail below, further constraints such as a        colour or intensity set point can be taken into account as well.        Therefore, in an embodiment, the method further comprises the        step of adjusting a duty cycle of the LED units in accordance        with the voltage output level while maintaining a colour set        point. As explained in more detail below, to accommodate for a        reduced input voltage, the LED assembly topology can be        adjusted. By adjusting the duty cycles of the LED units (thereby        e.g. reducing the intensity of the light output) a colour set        point can e.g. be maintained, even at a reduced voltage output        level.

The method of powering an LED assembly according to the first aspect ofthe invention can be generalised to a method of powering an LED assemblyconnectable to a power source, the LED assembly comprising two or moreLED units, the LED assembly further comprising a switch assemblycomprising one or more controllable switches for modifying a topology ofthe LED assembly, the LED assembly, in use, being powered by the powersource, the method comprising the steps of

-   -   detecting a voltage output level of the power source,    -   providing a control signal to the switch assembly to control the        switch assembly based on the voltage output level, thereby        modifying the topology of the LED assembly.

In an embodiment, the method, as described above, can be applied topower an LED assembly by a power source via an LED driver such as aswitched or linear regulator.

In an embodiment, the switch assembly includes switches forshort-circuiting the LED units.

In the lighting application according to the first aspect of theinvention, i.e. a lighting application comprising multiple LED units thepower factor can be improved by utilising multiple LED units andreducing the required converter output voltage (which is determined bythe forward voltages of the different LED units) by, temporarily,closing a switch over one or more LED units. By doing so, a largerportion of an AC input voltage can be applied for powering theconverter. Preferably, the operation of the switch assembly issynchronised with the supply voltage (e.g. an AC voltage or TRIAC dimmeroutput voltage). Such a synchronisation can include, as mentioned above,an appropriate selection of the duty cycle period.

According to a second aspect of the invention, there is provided an LEDdriver for powering an LED assembly that is suited for being suppliedfrom a dimmer circuit such as a TRIAC dimmer. The LED driver accordingto the second aspect of the invention comprises a converter forconverting a periodic input voltage to a supply current for powering theLED assembly, the LED driver further comprising a control unit arrangedto determine a minimal holding current by, in use, gradually reducingthe supply current until a value of the input voltage substantiallyreduces to zero and subsequently control the converter to operate at asupply current at least equal to the minimal holding current.

When an LED driver is being powered by a TRIAC dimmer or the like, it isimportant to ensure that the TRIAC remains in a conductive state duringthe time an output voltage needs to be provided. Such a TRIAC dimmer maye.g. convert an AC input voltage to a suitable (reduced) periodic inputvoltage (e.g. by phase or angle modulation) for conversion to the supplycurrent. In order to provide such a periodic input voltage, it may berequired to maintain the TRIAC in a conductive state. As will beacknowledged by the person skilled in the art, in order to maintain aTRIAC in a conductive state after triggering, a minimum current, alsoreferred to as the holding current, should pass through the mainterminals of the TRIAC. When the power demands of the LED assembly aresuch that the current as provided by the TRIAC dimmer is lower than theminimum value, the TRIAC leaves its conductive state. As a consequence,the input voltage as provided may become equal to zero. The LED assemblyaccording to the second aspect is arranged to ensure that, when acertain supply voltage is required by the LED driver, the LED driverload is such that a sufficiently high current is supplied by the dimmerto ensure the supply voltage being provided.

In an embodiment, the LED driver according to the second aspect of theinvention is applied in a lighting application, the lighting applicationcomprising an LED assembly comprising at least one LED, the lightingapplication further comprising a variable load, in use controlled by thecontrol unit, the variable load being connected in series with the LEDassembly.

As an example, the variable load can comprise a resistor and acontrollable switch (such as a FET or MOSFET) for short circuiting theresistor. Similar to the LED or LEDs of the LED assembly, the resistorcan be operated at a certain duty cycle thus requiring a certain amountof power from the LED driver. When the LED assembly (e.g. comprising aserial connection of a plurality of LED units such as the LED assemblyas applied in the lighting application according to the first aspect ofthe invention) is operating at a certain duty cycle (e.g. the LED unitsof the assembly operating at a certain duty cycle corresponding to a(user defined) set point of intensity and/or colour), a certain amountof power needs to be provided by the LED driver to the LED assembly. Inorder to supply this power to the LED assembly, this power needs to bereceived by the LED driver, e.g. from a TRIAC dimmer circuit. The LEDdriver according to the second aspect of the present invention isarranged to determine, based on a voltage measurement at the inputterminals of the LED driver, whether the power requested by the LEDdriver is sufficient to maintain a TRIAC of the dimmer circuit in aconductive state and, based on the voltage measurement, adjust the loadcharacteristic of the variable load.

In order to determine the required variable load for maintaining a TRIACof the dimmer circuit in a conductive state, a current vs. voltagecharacteristic can be determined e.g. during part of the periodic inputvoltage of the LED driver. Based on the characteristic, a minimum power(and current) can be determined in order to maintain the TRIAC in aconductive state.

The LED driver according to the second aspect of the invention enables asubstantially continuous assessment of the required (load) current tomaintain a TRIAC dimmer that can be used to power the LED driver in anoperating (conducting) state. As such, the LED driver is capable to,almost instantaneously, adapt to varying operating conditions and canensure an optimal (a.o. with respect to efficiency) operation of the LEDdriver.

In an embodiment, the load characteristic of the variable load asprovided in the LED driver is varied during part of the periodic inputvoltage of the LED driver while the voltage at the supply terminals ismonitored. When the load is varied to such an extent that the voltage atthe terminals drops to zero, the load as presented by the LED driver istoo small to maintain the TRIAC in a conductive state. Based on this, acontrol unit can determine a minimum load requirement for maintainingthe TRIAC in a conductive state.

In an embodiment, a substantially continuous assessment of the minimumrequired holding current for maintaining a TRIAC of an external dimmerin a conductive state can be obtained. As such, the LED driver iscapable of substantially continuously setting an optimal load current(i.e. a current sufficient to provide the required lighting output andsufficient to maintain the TRIAC in a conductive state.

In an embodiment, a trailing end of the periodic input voltage is usedto make the above mentioned load vs. voltage analysis. The outcome ofthe analysis can be applied during a subsequent period of periodic inputvoltage to set the required LED assembly load and variable load. Whenassessing the minimal holding current in a trailing end of the periodicinput voltage, the impact on the light intensity is comparatively smallor non-existent, as is explained in more detail below.

In an embodiment, the control unit of the LED driver according to thesecond aspect of the invention is arranged to

-   -   1. control the converter to reduce the supply current    -   2. measure a value of the input voltage at the reduced supply        current    -   3. repeat steps 1 and 2 during subsequent periods of the input        voltage until the supply voltage substantially reduces to zero,    -   4. controlling the converter to increase the supply current        during a subsequent period.

In order to power an LED unit, the LED driver according to the secondaspect of the present invention comprises a converter (e.g. a Buck orBoost converter) for converting an input voltage to a supply current forpowering the LED unit. Such an LED unit comprises at least one LED butmay equally correspond to an LED assembly as applied in the lightingapplication according to the first aspect of the invention. Theconverter as can be applied in the LED driver according to the secondaspect of the invention can e.g. correspond to a drive unit (or powerconverter) as e.g. applied in the lighting application according to thefirst aspect of the invention. So, either a switching regulator or alinear regulator could be applied.

As will be acknowledged by the skilled person, maintaining a certainholding current may result in an important power dissipation in theconverter. In order to mitigate such dissipation, the LED driveraccording to the second aspect of the invention comprises a control unitthat enables to find, e.g. in an iterative manner, which supply currentis required to maintain the input voltage. It can be noted that theminimum holding current may vary substantially, depending on theoperating temperature of the TRIAC. At −40 C, a minimum holding currentof approx. 30-50 mA may be required, while at +25 C an average TRIAConly requires 5-10 mA. In order to determine a minimum value of thesupply current (required to maintain the input voltage), the controlunit of the LED driver is arranged to control the converter to reducethe supply current and to measure a value of the input voltage at thereduced supply current. As long as the input voltage is maintained, thesupply current is sufficient to maintain the converter from providingthe input voltage. The control unit is further arranged to repeat thesteps of controlling the converter to reduce the supply current and tomeasure a value of the input voltage at the reduced supply current untilthe input voltage substantially reduces to zero, e.g. due to a TRIACaborting it's conductive state.

In an embodiment, the minimum holding current can be determinedsubstantially without being visually detectable as flicker. At the endof a period of the input voltage (e.g. synchronised to the line phase) atest can be performed to lower the supply current even further whilekeeping the overall holding current at a higher level with a hysteresisfor stability. Only losing power for a small part of the supply voltageperiod is something that can be corrected by a marginally bigger inputcapacitor (<5%).

In an embodiment, an input capacitance is provided to the LED driveraccording to the second aspect of the present invention. Such an inputcapacitance can be applied as a buffer for providing a supply voltage tothe LED driver when the supply voltage is comparatively low. Such acapacitance may equally serve as a filtering element.

In an embodiment of the LED driver according to the second aspect of theinvention, a comparatively small input capacitance can be providedbefore the power converter by making use of a non-linear transfer curvebetween the supply voltage (e.g. provided by a TRIAC dimmer) phase cutand the output level: When a TRIAC dimmer is provided to power an LEDdriver, an angle modulated AC voltage can be provided as the supplyvoltage. By applying a non-linear relation between the angularmodulation of the dimmer and the light output, e.g. a zero degrees phasecut resulting in 100% of the nominal light while an 90 degrees phase cutresulting in only 30% of the nominal light (rather than 50% of thenominal light), the input capacitance supplying the LED driver when thesupply voltage is comparatively low (or substantially zero due to theangle modulation) the capacitance can be almost a factor of 2 smaller.The user experience while dimming would remain substantially unaffectedby such a non-linear implementation, as the typical TRIAC dimmer has noscale and users operate a dimmer “until satisfied with the result”.

In an embodiment, the LED driver according to the second aspect of theinvention comprises an input buffer, e.g. a switchable capacitor, asexplained in more detail below.

In such an embodiment, it can be decided to limit the energy intake ofan input buffer, e.g. a capacitance to the time it takes to fill acapacitor or other storage element to the extend that it can power theremaining part of the cycle (this can e.g. be obtained by application ofa switchable storage element or buffer, as is explained in more detailbelow). This can be to the benefit of power efficiency.

When an input capacitance, in general a storage element or buffer, isapplied to power the LED driver during part of a period of the supplyvoltage, it may be cumbersome to determine a required dimming level asthe powering of the LED driver by the capacitance may result in a TRIACof a dimmer dropping out of its conductive state i.e. without anyadditional measures, such an implementation may prohibit determining thedimming level since this information is lost because of the TRIACswitching off for lack of holding current.

In order to resolve this, in an embodiment of LED driver according tothe second aspect of the invention, the control unit of the LED driveris arranged to maintain at least the minimal holding current during atleast an entire period of the supply voltage, e.g. once every 5, 10 or50 cycles. As an example, in case a 50 Hz AC voltage is provided asinput for a TRIAC dimmer, and the dimmer output voltage is subsequentlyrectified by a full bridge rectifier, a supply voltage having a periodof 10 ms is obtained. In this manner each 50, 100 or 500 ms the holdingcurrent (e.g. 50 mA) is maintained during an entire period thus enablingthe actual dimmer setting to be determined while still gaining 4/5, 9/10or 49/50 of the dissipation advantage of not having to hold the TRIACcurrent for an entire period of the supply voltage. In an embodiment,the LED driver according to the invention is powered from an electronictransformer. In general, such an electronic transformer converts aninput power source (e.g. an 230 V, 50 Hz mains supply) to a pulsed powersupply, e.g. providing a pulsed voltage of 11.5 V at 35 kHz. Inpractice, a plurality of LED units or LED assemblies are often poweredfrom a single electronic transformer. In such an arrangement, similarproblems can occur with respect to loss of power (i.e. the transformerceasing to provide power to the load) when the total power drawn by theplurality of LED assemblies descends below a minimum holding current ofthe electronic transformer. In general, an electronic transformer willattempt, when a certain period has expired, to output power again incase of a loss of power. Said period, e.g. 400 microseconds, may dependon the operating conditions prior to the loss of power. When the powerdrawn by the LED units or LED assemblies is too small, the outputvoltage of the electronic transformer can drop to zero. In order tomaintain the output voltage of the transformer to the required voltage,the control unit of the LED drivers/units or lighting applicationsaccording to the invention can be arranged to increase the powerconsumption of the LED drivers, by adding an extra load. However, as itis a-priori unknown with how many LED assemblies (say in a case: N) theelectronic transformer is loaded, the extra load may rise to N times theload which is minimally necessary for all types of electronictransformers to stay outputting power. This follows from the observationthat such an extra load can only be determined at design-time of the LEDdriver and/or LED assembly. With certain types of load (f.e.capacitive), such a high load may damage the electronic transformer,thus limiting N to only 1 or 2 nodes. By making the LED driver adaptingto the situation, that is to the number of LED assemblies N, a situationcan be reached that only the bare minimum of extra load is added overthe entire system (that is over all N LED assemblies) to keep theelectronic transformer alive.

Assuming the additional load needed to keep the electronic transformeroperating to be a capacitor of X nF. In case more than one lightingapplication is powered from the transformer, it may be sufficient to addto each lighting application a load which is only a fraction of X nF,namely substantially 1/N times X nF. As N is a priori unknown, it isproposed according to the invention, to gradually increase theadditional load whereby an assessment is made whether the added load issufficient, each time a load, e.g. X/Y nF is added. Each time a load isadded, the electronic transformer will, as indicated above, attempt tooutput power again. In case the load of the transformer is insufficient,the transformer will cease to output power indicating that furtheradditions of the load are required. As such, it may typically take a fewperiods before the total added load by the N LED assemblies equals orexceeds the minimal extra load. As an example, assuming a minimal loadrequirement to be 15 nF whereby the load represented by each LEDapplication can be increased in steps of 2 nF during each period. Insuch a situation, it would take three periods to obtain or exceed theminimal load when 6 LED assemblies are powered by the transformer. Incase 10 LED assemblies are powered, it would only take one period toobtain or exceed the minimal load. As soon as the minimal load requiredis added, the lighting applications can stop adding load. Using thisapproach, one can avoid that the total load to be powered by theelectronic transformer increases to a level that would cause damage tothe electronic transformer.

According to a third aspect of the invention, there is provided an LEDdriver for powering an LED assembly comprising at least one LED, the LEDdriver comprising a converter for converting a periodic input voltage toa supply current for powering the LED assembly, the converter havinginput terminals for receiving the periodic input voltage, the LED driverfurther comprising an input buffer for providing a current to theterminals and a control unit, the LED driver further comprising aswitching element for opening and closing a current path from the inputbuffer to the terminals and wherein the control unit is further arrangedto control the switching element based on an input signal representingthe periodic input voltage.

By applying an LED driver according to the third aspect of theinvention, a significantly smaller input buffer (e.g. a capacitor) isenabled by only connecting the capacitor to the periodic input voltage,e.g. a rectified voltage when either the rectified voltage is higherthan the current capacitor voltage value or the input voltage is too lowto supply the power convertor, it is therefore coupled to the line inputphase. Unlike the traditional filter capacitance e.g. applied with arectifier, the LED driver according to the third aspect of the inventionenables the capacitor's voltage to be kept high until the stored poweris actually needed instead of the capacitor's voltage decreasing withthe input voltage decreasing. Using this method, an input capacitor sizecan for example be reduced by a factor of 10.

In an embodiment of the LED driver according to the third aspect of theinvention, the switching instances or moments of the power convertor aresynchronised with the line phase. Due to the synchronisation a morereproduce-able LED current flow is achieved while EMI is reduced sincethe switch moment can be chosen optimal for this. In addition, since the50, 60, 400 or 480 frequencies are very precise over multiple cycles,the synchronisation of all switching and feedback elements to the linephase in an intelligent mains voltage LED driver leads to a more stablelight output regardless of temporary variations which are abundant on anaverage mains supply.

As an example of such a synchronisation, the selection of the duty cycleperiod relative to the supply voltage period as described above, can bementioned.

In an embodiment, the input buffer as applied comprises a capacitorassembly comprising a plurality of capacitors and wherein the switchingelement is further arranged to control a topology of the capacitorassembly based on the input signal. As an example, the capacitorassembly comprises two capacitors whereby the switching element isarranged to connect the capacitors either in series or in parallel. Byconnecting the capacitors in parallel when charging, the charging can bedone with a comparatively low voltage. When the charged capacitors aresubsequently connected in series, a comparatively large voltage becomesavailable for powering the converter and thus the LED assembly. In orderto modify the topology of the capacitor assembly or modify theinterconnection of the capacitor assembly to either the periodic inputvoltage or the power converter, the switching element as applied in theLED driver according to the third aspect of the invention can comprisemore than one switch. By controlling the switching element based on aninput signal representing the periodic input voltage, it is possible tochoose the time of connecting the capacitors for charging at timesenabling drawing a current from the supply that is larger than theminimum current needed to keep an electronic transformer or a TRIACdimmer alive during a needed amount of time. As such, by using severalcapacitors in a capacitor assembly, the charging of the capacitors canbe performed at times when the power requirements of the load areinsufficient to maintain an electronic transformer or a TRIAC dimmerproviding an output power. As such, the capacitor or capacitor assemblyas described, in general the input buffer, can be used as a variableload as applied in an embodiment of the LED driver according to thesecond aspect of the invention.

In another embodiment of the LED driver according to the third aspect ofthe invention, a smaller storage element or buffer (e.g. a capacitor) isenabled by introducing an on/off duty-cycle to the current provided bythe LED driver to an LED assembly, the on/off duty cycle beingsynchronised with the supply voltage (e.g. an AC mains voltage or TRIACdimmer output voltage). Preferably, the on/off duty cycle has afrequency content above 500 Hz to reduce flicker and nausea effects. Theoff-part of the duty-cycle can be chosen to align with the line phasewhere the capacitor is feeding the power convertor. Using this method,e.g. a 75% duty-cycle at 500 Hz would allow halving the capacitor in anexample case.

In an embodiment of the LED driver according to the third aspect of theinvention, the periodic input voltage which, in use, is applied to theconverter (or drive unit) is a TRIAC dimmer output voltage. In such anarrangement, the control unit of the LED driver should be arranged todetermine a required dimming level for the LED assembly in accordancewith the TRIAC dimmer output voltage This can e.g. be done bydetermining an average value of the supply voltage over a predeterminedperiod of time. Based on the determined dimming level, a set point canbe determined by the control unit for powering the LED units (e.g. aduty cycle of the LED units) for obtaining the required dimming level.

In an embodiment, the required dimming level is derived from theavailable voltage at the end of a boost stage (i.e. a discharge stage orphase of the input buffer discharging its energy into the load, thuspowering the load). This available voltage can be considered a measurefor the average available voltage and thus of the required dimminglevel.

As an alternative, the required dimming level can be derived from theoperation of the converter. This can be illustrated as follows: assumingthe converter as applied in the LED driver is a switched mode powersupply such as a Buck or Boost converter. Such a converter is, ingeneral, controlled to maintain a substantially constant output currentfor powering the LED assembly. In order to maintain such a constantoutput current, a switching element of the converter will operate at acertain duty cycle. In case the input voltage of the converter wouldchange, this change would affect the duty cycle of the switchingelement. A larger input voltage would require the switching element tooperate at a smaller duty cycle in order to maintain the output current.This mechanism can be applied to adjust the brightness of an LEDassembly in the following way. Rather than maintaining the outputcurrent to a substantially constant level, the converter of the LEDdriver according to the invention is controlled in such way that aswitching element of the converter is operating at a substantiallyconstant duty cycle or within a duty cycle range. An increase of theinput voltage would lead to a smaller duty cycle of the switchingelement. By, in response to such a smaller duty cycle, setting thebrightness set point higher (and/or change the topology of the LEDassembly), the power drawn from the regulator/drive unit is increasedcausing the duty cycle to increase again. So by changing the set pointof the brightness (and thus the dimming level), the duty cycle can bekept substantially constant, and the dimming level observed will followthe incoming average voltage level and thus the TRIAC dimmer setting.

In such an embodiment, no additional hardware such as an ADC (analogueto digital converter) for providing a signal to the control unitrepresenting the input voltage is needed. Generalising this principle,the duty cycle at which a switch of the power converter of the LEDdriver is operating, can be considered a measure for the availablesupply voltage or the difference between the available supply voltageand the required load voltage and can thus be applied to control thetopology of the LED assembly, by controlling the switch assembly.

In another embodiment of the LED driver according to the third aspect ofthe invention, a smaller capacitor is enabled by introducing acurrent-setting duty-cycle whereby the current supplied to the powerconverter by the capacitor during part of the input voltage period islower than the current supplied when the capacitor is not discharged tothe power converter. Phrased differently, the control unit of the LEDdriver can be arranged to control the LED driver to apply a reducedcurrent to the LED assembly when the LED assembly is powered from theinput buffer. Using this method a reduction of the input buffer (e.g. acapacitor) of up to 30-40% can be achieved without negative impact tovisible flicker. Preferably the current setting is coupled to the lineinput voltage. In such an arrangement, in order to maintain a certainbrightness, a larger current or an increased duty cycle can be appliedwhen the power converter is not supplied from the buffer.

In another embodiment of the LED driver a significantly smaller inputcapacitor is enabled by charging a larger capacitance at the output ofthe power convertor which can be discharged when the rectifier outputvoltage is insufficient for the power convertor. In an additionalembodiment, the voltage stored is doubled by a diode/capacitor networkbefore being used.

As mentioned above, the switchable input buffer can e.g. comprise acapacitance or an inductance. By combining both in an LED driver, theLED driver can be arranged to operate as either a substantiallycapacitive load or a substantially inductive load. By alternatingoperating the LED driver as a capacitive load, e.g. during multipleperiods of the input voltage (thereby applying the capacitance as inputbuffer) and as an inductive load e.g. during multiple periods of theinput voltage (thereby applying the inductance as input buffer), thepower factor of the LED driver can be adjusted and improved. In casemultiple LED drivers are applied, e.g. in a lighting applicationcomprising multiple LED assemblies, each powered by an LED driver, apower factor compensation or adjustment can be realised by applying bothLED drivers having a capacitive buffer and LED drivers having aninductive buffer.

In an embodiment, the LED driver according to the third aspect of theinvention comprises an EMI filter comprising a filter capacitorconnected to the terminals, in parallel to the switched input buffer. Inan embodiment whereby the input buffer comprises a capacitor connectableto the terminals by the switching element, an additional switch isprovided in a conductor between the filter capacitor and the capacitor.By appropriate operation of the additional switch such that thecapacitor is not charging the filter capacitor, an improved behaviourwith respect to audible noise and EMI can be obtained as is explained inmore detail below. Applying the switch in a ground wire or conductorrather than in a live wire, the control of the switch can befacilitated.

Further details and advantages of the LED drivers and lightingapplications according to the present invention are provided in thedescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematically depicts a first embodiment of the lightingapplication according to the first aspect of the invention.

FIG. 1 b schematically depicts a second embodiment of the lightingapplication according to the first aspect of the invention.

FIG. 2 a schematically depicts a third embodiment of the lightingapplication according to the first aspect of the invention.

FIG. 2 b schematically depicts two possible arrangements of an LEDassembly directly powered from a rectified supply voltage.

FIG. 3 a schematically depicts the required forward voltage when threeLED units are powered at different duty cycles, as a function of time.

FIG. 3 b schematically depicts how to power a plurality of LED unitswhen a variable supply voltage is available.

FIG. 4 schematically depicts how a plurality of LED units can be poweredby a substantially sinusoidal voltage.

FIG. 5 schematically depicts an embodiment of an LED driver according toa second aspect of the present invention.

FIG. 6 schematically depicts a lighting application according to theinvention comprising an embodiment of an LED driver according to thesecond aspect of the invention.

FIG. 7 a schematically depicts a TRIAC dimmer output voltage and timeintervals for assessing a minimal holding current for the TRIAC.

FIG. 7 b schematically depicts controlling a time interval wherein aminimal holding current is supported.

FIG. 8 schematically depicts a possible effect on light output whenpowering an LED driver by a rectified sinusoidal voltage when no inputbuffer is applied.

FIG. 9 schematically depicts voltage and current waveforms of an LEDdriver applying a switchable buffer.

FIG. 10 schematically depicts a first embodiment of a switchable bufferas can be applied in the present invention.

FIG. 11 schematically depicts a second embodiment of a switchable bufferas can be applied in the present invention.

FIG. 12 schematically depicts the switchable buffer of FIG. 11 applyingswitches rather than diodes.

DESCRIPTION

The present invention provides, in a first aspect, a lightingapplication comprising a plurality of serial connected LED units whichcan e.g. be powered from a dimmer output signal, in general, from apower supply source. FIG. 1 a schematically depicts a first embodimentof the lighting application according to the invention. FIG. 1 aschematically depicts a serial connection of three LED units 10, 20 and30. The embodiment further comprises a switch assembly comprising threeswitches T1, T2 and T3 that can substantially short circuit therespective LED units 10, 20 and 30. The switches can e.g. comprise a FETor a MOSFET. FIG. 1 a further depicts a drive unit (also referred to aspower converter) 50 for powering the LED units and a control unit 40 forcontrolling the drive unit 50. The drive unit can e.g. be, as shown inFIG. 1 a, a buck converter or can be another type of converter thatenables the application of a current I to the LED units. The drive unit50 is supplied from a voltage source V. In general, such a buckconverter is supplied from a substantially constant DC voltage. When thesupply voltage V is not constant (e.g. comprising an AC component), thesupply voltage may, at some instances, be insufficient to power the LEDunits as the LED units each require a certain forward voltage Vf. Inorder to enable the application of a varying supply voltage as input fora drive unit for an LED based lighting application, the lightingapplication according to the invention is provided with a switchassembly and is arranged to provide a signal 60 to the control unit 40,the signal representing the supply voltage V that is provided to theconverter 50. The signal 60 can e.g. represent a property of the supplyvoltage V such as a voltage level or a zero-crossing instance or cane.g. the voltage difference between the supply voltage V and the loadvoltage Vf. As shown in FIG. 1 a, the control unit 40 can further beequipped to provide an On/Off signal to the converter 50 in order toturn the current source on or turn it down. The control unit 40 mayfurther be arranged to control the switching element T of the converterby providing a control signal S to the drive unit 50. Also a voltageover resistance Rs (representing the current through the LED assembly)can be applied as a feedback to the control unit 40 and to the converter50 (inputted at a terminal FB of the converter) and can be applied tocontrol the switching element T of the converter or the switch assembly.

The signal 60 as provided to the control unit and representing at leasta property of the supply voltage V can be applied by the control unit todetermine a required dimming level (e.g. in case the supply voltageoriginates from a TRIAC dimmer circuit). This can e.g. be done bydetermining an average value of the supply voltage over a predeterminedperiod of time. Based on the determined dimming level, a set point canbe determined by the control unit for powering the LED units (e.g. aduty cycle of the LED units) for obtaining the required dimming level.In this respect, it is worth noting that in some lighting applicationswhich comprise multiple light sources such as LED assemblies, it isconsidered important that each light source provides at the sameintensity and/or the same colour. In order to achieve this in a lightingapplication comprising multiple LED assemblies, it is thus importantthat each LED assembly is operated at substantially the same set pointfor intensity and/or colour. As the required set point is e.g. derivedfrom an input signal (e.g. signal 60 as shown in FIG. 1 a), it may occurthat due to tolerances, the control units of the multiple LED assemblyderive a different set point from the input signal. In order to overcomethis, it can be arranged to assign a set point (either an intensity orcolour set point) corresponding to the input signal when the inputsignal is within a certain bandwidth or margin. It will be clear to theskilled person that, by doing so, a trade-off is made between theresolution that can be realised with respect to colour and/or intensityand the requirement to have the same output with respect to colourand/or intensity. It can further be noted that this way of deriving aset point for an intensity and/or colour of an LED assembly may also beimplemented in control units as applied in LED drivers according to thesecond or third aspect of the invention.

As an alternative, it can be arranged that the control unit of one ofthe LED assemblies operates as master and derives an intensity and/orcolour set point of the input signal representing the supply voltage andprovides the set point to the other control units controlling the otherLED assemblies of the multiple LED assemblies. By doing so, an improvedresolution of the intensity and/or colour can be maintained whileensuring that the same set point is applied by all LED driversassociated with the multiple LED assemblies.

The switch assembly allows changing the topology of the serial connectedLED units by short circuiting one or more of the switches T1, T2 and T3.In the embodiment as shown, each LED unit can be shorted by turning onan appropriate switching element, which can e.g. be controlled by thecontrol unit 40, thereby effectively lowering the minimum voltage inputVf required by the drive unit 50 to still provide a current to theremaining LED units. In order for the control unit 40 to determine thenumber of LED units that can be powered, the control unit 40 is arrangedto receive a signal representing the available voltage for powering theLED units. Such a signal can e.g. be obtained directly from the driveunit input voltage V. Based on the available voltage for powering theLED units and the required voltage by the different LED units, thecontrol unit can determine which topology or topologies can be poweredby the available voltage. The control unit may then control theswitching assembly in such manner (e.g. by controlling the switches thatbridge the LED units) that the required topology is obtained. Reducingthe number of LED units that are on in the low voltage ranges of thesupply voltage V of the drive unit 50 allows e.g. a buck convertor tosupport a larger range of the supply voltage, especially interesting for(temporarily) lowering required output voltage depending on availableinput voltage.

The embodiment as shown in FIG. 1 a further comprises a diode 70 andcapacitor 75 connected between the supply voltage V and the LEDassembly. In FIG. 1 a, the capacitor is connected to a node between theLED units 10 and 20 but may equally be connected between other LED unitsor connected to ground. As will be acknowledged by the skilled person,the capacitor 75 as implemented can be charged by the supply voltage V.As such, it provides a voltage source that, in case the supply voltage Vis small or zero (in case the supply voltage e.g. comprises an ACcomponent or comprises a rectified AC voltage or a TRIAC dimmer outputvoltage) can be applied for various purposes. The available voltage cane.g. be applied to drive the switches T1, T2 and T3 in case the supplyvoltage V is too low. The available voltage can also be applied as asupply voltage for the control unit. As the control unit, only requiresa small operating current (˜1 mA), a comparatively small capacitor maysuffice to temporarily supply the control unit when the supply voltage Vis too low. When a comparatively large capacitor 75 is applied, thestored energy may even be sufficient to power the LED units during acertain period when the supply voltage V is too low.

The embodiment as shown in FIG. 1 a and the embodiments discussedfurther on can, optionally, be provided with a rectifier or a rectifierelement. Such a rectifier can rectify the incoming AC waveform (i.e. thedimmer output voltage) thereby e.g. generating a pulsed DC waveform thatcan be used to supply the drive unit of the lighting application. In itssimplest form it consists of a single diode which then leads to a singlephase output with a large off-period. A further embodiment can e.g.comprise a diode bridge comprising 4 diodes that use both AC phases andcan lead to a fully rectified output. Due to the minimum forward voltagedrops the diodes cause some current and voltage distortion and alsoaccount for some dissipation. Most of the dissipation and distortion canbe removed by using low voltage drop switching elements (e.g. FETs)instead of the diodes. In applications using a comparatively smallvoltage (e.g. 12 or 24 V AC), the application of switching elements mayprovide an important gain in available voltage.

In an embodiment, the lighting applications as depicted in FIG. 1 acomprise a linear regulator as a drive unit instead of a switchingregulator such as the Buck converter shown. Due to a comparatively lowefficiency of such a converter, it is important to maintain thedifference between the supply voltage V and the load voltage Vf as smallas possible. The switch assembly as applied in the present invention canbe used for this. Based on the signal 60, the control unit CU candetermine an optimal configuration for the LED assembly such that amismatch between the supply voltage V and the load voltage Vf is assmall as possible.

In an embodiment, the drive unit of the lighting application furthercomprises a so-called current mirror combined with a linear regulator.Such an arrangement is schematically depicted in FIG. 1 b. FIG. 1 bschematically depicts an AC supply voltage 81 connected to a rectifier82 thus obtaining a supply voltage Vsup. The arrangement furthercomprises a regulator REG 83 arranged to supply a current Iref to oneside of the current mirror 84 comprising two transistors Ta and Tb in awell known mirror arrangement. Using such an arrangement, the currentIled through the LED assembly 85 can be controlled to the same value asIref. FIG. 1 b further shows a control unit 40 (comparable to thecontrol unit as shown in FIG. 1 a) receiving a feedback signal Vicerepresenting a voltage difference between the supply voltage Vsup andthe voltage over the LED assembly 85. This feedback signal can beapplied by the control unit 40 (in a similar manner as described withrespect to FIG. 1 a) to control the switch assembly comprisingcontrollable switches T1, T2 and T3.

FIG. 2 a schematically depicts another embodiment of the lightingapplication according to the first aspect of the invention. The lightingapplication as shown comprises an LED assembly 110 comprising aplurality of LED units and a switch assembly SA 106 arranged to, a.o.short circuit one or more of the LED units, in general, modifying thetopology of the LED assembly, as discussed above. The embodiment furthercomprises a controller (or control unit CU) 108 arranged to control theswitch assembly 106 and the power converter (or drive unit DU) 104 that,in use, powers the LED assembly 110. Reference number 99 denotes, ingeneral, the voltage as provided to the lighting application. As such,it can e.g. correspond to an AC mains voltage or a dimmer output voltage(e.g. a TRIAC dimmer output voltage).

In an embodiment, the lighting application can comprise an EMI Filter(100) which can be applied to comply, if required, with legislaturerequirements on EMI, common-mode, and differential mode filtering. Whenswitching effects of the power convertor (104) are not strong enough torequire attenuation of the signal level to meet regulatory emissionrequirements, it can be omitted. Such an EMI filter can e.g. comprise aninductor and capacitor (LC) filter to suppress a switching frequency ofthe power convertor 104. In an earthed situation, a common mode chokecan be used e.g. consisting of two distinct coil windings on a singlecore in combination with a small capacitance. Various examples of EMIfilters both for earthed and non-earthed situations are known and can beapplied in a lighting application or in combination with an LED driveras provided by the invention.

In the embodiment of the lighting application as shown in FIG. 2 a, thelighting application further comprises a waveform analyser WA 112. Sucha waveform analyser 112 is intended to provide information on theavailable voltage to the controller 108. Such information may e.g.relate to the voltage level available and/or the timing when the voltageis available. As an example, such a waveform analyser can e.g. samplethe available voltage at a certain rate and store the information, e.g.after an A/D (analogue to digital) conversion. The waveform data mayalso be retrieved via a comparator, e.g. a comparator having multiplelevels thus retrieving information when a certain voltage level isexceeded or not. In an embodiment, such a waveform analyzer can e.g.enable a synchronisation between the supply voltage that is provided asan input voltage to the drive unit or to the rectifier and the controlunit thereby enabling the control unit to synchronise its controlactions with the available input voltage. As an example, the waveformanalyser can, preferably over multiple cycles, determine an as accurateas possible synchronisation of the control unit to the supply voltage,e.g. to the line and/or (electronic) transformer frequency. This can forexample be done by (software) PLL locking to compensate for cycle-localline voltage distortions. The control unit may then e.g. base itscontrol moments in time on the line phase detection. In an embodiment,the waveform analyser is arranged to detect a zero crossing of thedimmer output voltage. By doing so, a synchronisation between the dimmeroutput voltage and the control unit controlling the drive unit can berealised. As an example, the dimmer output voltage as can be applied topower the drive unit can e.g. correspond to an AC voltage which isdimmed using a TRIAC based dimmer and rectified. In order to know whichvoltage is available at a given moment in time, the available voltagecan be sensed by the waveform analyser and a zero crossing can bedetected. The waveform analyser may further be arranged to provide asynchronisation signal to the control unit based on the detected zerocrossings. In case of a TRIAC based dimmer, the firing times of theTRIAC (which can e.g. be expressed by a modulation angle) may furtherprovide information regarding the available voltage for powering the LEDunits. When the dimmer input voltage is known (e.g. 230 V, 50 Hz), adetection of the zero crossings and information regarding the appliedmodulation angle may provide sufficient information to the controller orcontrol unit 108 to determine/predict, at any given time, the availablevoltage. As an alternative to the application of zero-crossings tosynchronise the control unit 108 operation with the supply voltage, adetection of a peak value (or top value) of the supply voltage canequally be applied. Such peak value detection can e.g. be implemented bysampling the supply voltage (e.g. by using an A/D converter), storingthe results of n (e.g. 10) latest samples and determining whether a peakvalue is observed within the n samples.

In the example as shown, the prediction of the available voltage relies,to a certain extend, on the voltage shape (e.g. a sinusoidal shape)being preserved when the TRIAC is conducting. In case the dimming actionresults in a less predictable voltage shape, the available voltage forpowering the LED units can e.g. be derived in the following manner. Inaddition to or as an alternative to the detection of the zero crossings,the waveform analyser 112 can be arranged to register and store thewaveform into a memory unit. When the waveform has been stored, thewaveform analyzer 112 may thus provide the control unit 108 withinformation of one or more of the previous cycles. The waveform analyser112 can e.g. store in a memory unit where in time which voltage wasavailable (e.g. as a time-voltage curve or table) which can be used todetermine which moments in time are effective in the next cycle to usefor actual power supply to an LED assembly having a particular LEDtopology (which may e.g. be adjusted based on the assembly forwardvoltage requirements). Based on such a time-voltage curve or table, thecontrol unit 108 can determine the maximum number of LED units that canbe powered or which LED units can be powered at a given time. It will beappreciated that such an arrangement does not pose any limitation on thewaveform shape provided that the voltage should have a substantiallyrepetitive nature. In case of a rectified (e.g. by a full bridgerectifier) AC voltage that is subsequently altered by a dimmer circuit,the dimmer output voltage may e.g. have a substantially non-sinusoidalshape but still periodically; the waveform may e.g. repeat at a 100 Hzrate in case of a 50 Hz AC input voltage. In order to determine the rateof repetition of the waveform, the detection or determination ofzero-crossings of the supply voltage can e.g. be applied.

In general, the waveform analyser 112 provides information on theavailable voltage to the control unit 108, in order for the control unitto determine which and/or how many LED units can be powered.

The information on the waveform can be retrieved from a number of accesspoints on the lighting application. Depending on the access pointapplied, the information that can be retrieved may vary. As an example,in case only zero-crossings of the voltage available to the LED unitsneed to be determined, these zero-crossing can e.g. be determined fromthe input voltage of the drive unit or, in case a rectifier REC 102 isapplied, even from the voltage 99 that is supplied to the rectifier,e.g. an AC mains or TRIAC dimmer output voltage.

In an embodiment, the lighting application is provided with a filtercausing a delay between the dimmer output voltage and the drive unitinput voltage. Such a delay, e.g. provided by a filtering capacitance,can be applied to determine the voltage available to power the LED unitsin advance.

The waveform analyser 112 can e.g. acquire an input voltage of atransformer with some limited filtering (to reduce input capacitance) inone 100 Hz (or 120 Hz, or 400 Hz, etc.) cycle and map the input voltageto an appropriate voltage and current domain to be used in the next 100Hz cycle: e.g. by determining for each 1 ms the voltage available of a12 V AC output, an rms voltage between 0 and 12 V is found. The nextcycle in that particular 1 ms period the maximum # of LEDs that can bepowered by the available voltage level can be turned on. In this mannerthe output voltage (of the converter 104) may substantially follow theavailable input voltage over time with some margin. This may provide ina, for the human eye stable light output at comparatively low inputcapacitor values, thus avoiding the use of electrolytic capacitors whichtend to be comparatively big, costly, and have a limited life time.

In the embodiment shown, the lighting application may further comprisean input filter and a switched buffer SB1 103 as indicated in FIG. 2 a.Such optional element may serve to supply the drive unit (or powerconverter) 104 when an output voltage of the dimmer circuit or therectifier has fallen below a minimum level usable for the drive unit(104). The optional input filter may comprise a fixed input filter,operating as a buffer, which can be connected directly to the rectifiedvoltage. The embodiment as illustrated may further comprise a secondary(optional) buffer arranged to momentarily connect a buffer element (e.g.a capacitance) to the rectified voltage thereby charging the buffer and,at a later time, connect the buffer element to the input of the driveunit (also referred to as the power converter 104) thereby dischargingthe buffer in order to supply power to the power convertor when therectified voltage has fallen too low. Further details and embodiments ofsuch a switched buffer are disclosed below. The advantage of providing aswitchable buffer is to save the stored high voltage in the buffer foroptimal use at the time it is needed. The timing of connecting anddisconnecting such a switchable buffer can be done autonomously by e.g.always accepting charging and saving discharging only when the inputvoltage of the power convertor 104 drops below a minimum level. Apreferred embodiment however would e.g. enable the controller (108) tocontrol the (dis)-connecting of the switchable element and e.g.synchronise the (dis)-connection with a line phase thus enabling a morerobust buffer charging during each power cycle independent frommomentary distortions of the line voltage. The buffer (or storage)element normally comprises a capacitor but can also take the form of aninductor.

The drive unit 50 (of the embodiment of FIG. 1 a) or the drive unit 104(of the embodiment of FIG. 2 a) as applied in the lighting applicationaccording to the invention can e.g. be a buck regulator out of cost,efficiency and size considerations, but can also consist of abuck-boost, boost, SEPIC, CUK, etc., or any multiples or anycombinations thereof. Instead of a switching regulator such as a Buck orBoost regulator, a linear regulator can also be applied as a drive unit50.

The embodiment of the lighting application as shown in FIG. 2 a mayfurther, optionally, comprise an output filter and switched buffer SB2105. The optional output filter can be used to reduce current andvoltage ripple to the connected LED load, i.e. the LED assembly 110 andcan e.g. comprise a capacitor. The optional switched buffer is an energystorage element that can be used to store energy during sufficientrectified voltage being available and can be tapped during the intervalthat the rectified voltage is not sufficient. The stored energy can befed back to the input of the power convertor 104 when needed.

In an embodiment, the drive unit switching element or elements (e.g.switch T of the drive unit 50 of the embodiment as shown in FIG. 1 a)can e.g. be synchronised to the line phase under control of thecontroller (108) and can e.g. comprise FETs, bipolar transistors, etc.The drive unit may use capacitors or inductors as storage elements thatare switched. The power convertor may be direct off-line (notgalvanically isolated) or galvanically isolated by e.g. a flybacktransformer and accompanying switching element.

The control unit or controller (108) is the component of the lightingapplication that, based on a supply voltage based input signal (e.g.provided by the waveform analyser (112)) may align its control actionsrelative to the actual line phase. To enable this, the input signal tothe controller may e.g. comprise synchronisation information such as thetiming of the zero crossings of an AC input voltage. The controller orcontrol unit 108 may optionally control the switched buffer 103 and thusenable a reduction of the buffer and filter capacitor's size:Constraints on the controlling of the switched buffer and the requiredavailable supply voltage can be applied as design input data to e.g.choose the capacitor size. This is explained in more detail below. Thecontroller 108 may optionally control the power convertor 104 to reducelight output flicker caused by momentary line voltage fluctuations, itcan (optionally) also set an on/off duty-cycle over time for theconverter. The controller 108 may optionally control the output switchedbuffer (105) to allow storing energy when the supply voltage issufficiently high for use when the voltage is insufficient to power theLED assembly. The controller 108 may also control the LED topologyswitches or switch assembly 106 to set a mix-colour point, or reduce aninput voltage requirement of the power convertor 104 by reducing the #of LED units on. This is e.g. achievable by substantially shortcircuiting one or more of the LED units (by the switch assembly 106)thereby reducing the required forward voltage of the LED assembly 110.In the embodiment as shown in FIG. 2 a, the controller 108 may take asinput a feedback value 109 that may be use to achieve a light outputcolour and/or brightness set point. The controller 108 may furthermorebe given a software algorithm such that it optimises power efficiency,light output or colour stability, and sufficient power factorcorrection. The controller 108 may use the waveform analyser's 112 dataon a previous cycle waveform voltage data to set & optimise powerconversion for the next cycle. The feedback 109 may give the controller108 feedback on the actual light output and/or colour and may e.g.comprise a current sensing resistor or other method of measuringcurrent. Another embodiment would be an optical feedback using an LEDand photodiode as feedback in e.g. a galvanically isolated flybackconvertor. Another embodiment would be to use direct light outputfeedback from a suitably mounted photo diode or other optical sensor.

In an embodiment, the control unit 108 can further be arranged todetermine an average of the dimmer output voltage. Such average may e.g.be applied by the control unit to determine a set point for theintensity to be realised by the lighting application. In a retrofitapplication of the present invention (where a conventional lightingapplication is replaced by a lighting application according to thepresent invention), this can be applied to mimic the response of thereplaced lighting application to a dimmer action. When a filtering isapplied to the dimmer output voltage, the filtered voltage may directlybe applied as a signal representing the average dimmer output voltageand may thus be applied to determine an intensity set point for thelighting application.

The LED assembly 110 as shown in FIG. 2 a can e.g. be a network ofserial connected LED units wherein each LED unit can comprise paralleland/or serially connected LEDs.

In an embodiment, the waveform analyser 112 may continuously analyse theincoming, e.g. (electronic) transformer's output, waveform in order torecover a line frequency (with the purpose of synchronising a controller108 to it) and may also determine over a (synchronised) e.g. 10 mssupply voltage period (in case of 50 Hz) when and which number of LEDunits can be powered by the drive unit 104, e.g. a buck convertor.

The application of a waveform analyser 112 that can retrieve and storeinformation on a previous cycle waveform voltage and supplies theinformation to a controller 108, can facilitate the controller incontrolling both the intensity and colour as generated by the lightingapplication.

According to the present invention, there is also provided an embodimentof an LED based lighting application which is more directly suppliedfrom an AC or periodic supply voltage. Rather than using a powerconverter or drive unit such as a switched mode power supply or a linearregulator, the lighting application according to the embodimentcomprises an LED assembly comprising two or more LED units, each LEDunit comprising one or more LEDs (such as LED units T1, T2 and T3 asshown in FIG. 1 a) whereby the LED assembly is powered from a periodvoltage, e.g. a rectified AC voltage. The LED assembly furthercomprising a switch assembly comprising one or more controllableswitches for modifying a topology of the LED assembly and a control unitcomprising an input terminal arranged to receive a signal representing avoltage level of the supply voltage or a load current of the LEDassembly, and an output terminal for providing a control signal to theswitch assembly to control the switch assembly in accordance with thesignal, thereby modifying the topology of the LED assembly.

Instead of electrically connecting a power converter between a supplyvoltage (e.g. voltage V as shown in FIG. 1 a) and the LED assembly, theLED assembly is powered from the supply voltage without the use of sucha converter. Instead, the control unit of the lighting application isarranged to adjust the topology of the LED assembly such that the loadvoltage substantially matches the supply voltage. Compared to thearrangement shown in FIG. 2 a, the power converter 104 can thus beomitted. In order to control the switch assembly and thus controllingthe load voltage, the control unit can receive, similar to thearrangements shown in FIG. 1 or 2, a signal representing the supplyvoltage (e.g. a rectified AC voltage) and control the switch assemblybased on this signal.

As a more direct approach, the control unit can rely its controloperations on a signal representing the load current, i.e. the currentprovided to the LED assembly. Based on the current provided to the LEDassembly, i.e. the load current, the control unit of the lightingapplication can determine how the topology of the LED assembly shouldbe. As an example, the LED assembly can comprise a serial connection ofn LEDs, each LED being provided with a parallel switch enabling ashort-circuiting of the LED (as e.g. shown in FIG. 1 a), whereby the LEDassembly is e.g. powered from a rectified AC voltage. Starting from aninstance whereby the supply voltage is substantially zero, 1 LED can beswitched on (by opening the switch of the switch assembly that isparallel to the LED. By doing so, a current will flow through the LEDaccording to the well known forward voltage (Vf) vs. current (I)characteristic of an LED. When the available supply voltage exceeds thenominal Vf of the LED, an important increase of the current through theLED (according to the Vf vs, I characteristic) can be observed. Thecurrent through the LED can e.g. be derived from a feedback signal e.g.originating from a sense resistor, such as resistor Rs in FIG. 1 a. Asignal representing the load current (i.e. the current through the LED)could also be derived from an optical sensor as the intensity of thelight generated can be considered a measure for the current through theLED. Based on the signal representing the load current, the control unitof the lighting application can e.g. in case the current exceeds acertain value, control the switch assembly to open a switch in parallelto a second LED thereby obtaining two series connected LEDs. As aresult, the available voltage will be distributed over both LEDsresulting in a reduced load current. Continuing this process, thecontrol unit can e.g. control the load current to remain within acertain bandwidth by adding or removing LEDs to/from the LED assembly.

As will be understood by the skilled person, adding or removing an LEDas described can result in an important current variation when only afew LEDs are operated. In order to mitigate this, in an embodiment, theLEDs or LED units of the LED assembly are provided with a parallelcapacitor which can be switched on or off by a switch connected inseries with the capacitor.

FIG. 2 b schematically depicts two possible arrangements enabling thecurrent variations due to adding or removing LEDs to be reduced. In thearrangement shown on the left of FIG. 2 b, the LED current is controlledby a switch Tx in parallel to the LEDs. The LEDs Lx are further providedwith a capacitance Cx which can be connected in parallel to the LEDs byoperating switches Ty. In such an arrangement, by closing switch Ty ofthe LED which is to be added, the capacitor in parallel to the LED ischarged. Switch Tx is assumed to be opened at the same time as Ty. Bydoing so, the voltage over the LEDs already operating can be maintainedsubstantially constant despite an increase of the supply voltage, byappropriate dimensioning of the capacitors Cx. Phrased differently, thevoltage over the capacitor can be designed to increase (due to thecharging of the capacitor) with substantially the same slope as thesupply voltage increases. As such, the voltage over the LEDs alreadyoperating can be maintained substantially constant. Once the capacitorhas been charged to e.g. the Vf of the LED to be added, the capacitorcan be switched off, i.e. switch Ty can be opened. During a decrease ofthe supply voltage, the charge stored in the capacitor can be appliedto, in a similar manner, maintain the voltage over the operating LEDs orLED units substantially constant. An alternative arrangement is shown onthe right of FIG. 2 b, whereby a switch Tz is provided for connecting anLED Lx, which can either connect a terminal 0 to either terminal 1(thereby disconnecting the LED Lx), terminal 2 (thereby connecting acapacitor Cx) or terminal 3 (thereby connecting the LED Lx). When theswitch Tz is connecting terminal 0 to terminal 1, LED Lx is not providedwith a load current. In case the available supply voltage increases, theswitch Tz of an LED Lx to be added is operated to, in a first step,connect terminal 0 to terminal 2 thereby charging the capacitor Cx.Similar to the arrangement on the left, charging the capacitor canresult in the voltage provided to the operating LEDs to remainsubstantially constant. Once the capacitor is charged, e.g. to theforward voltage Vf of the LED to be connected, switch Tz can be operatedto connect terminal 0 to terminal 3, substantially without causing anycurrent variations.

It is worth noting that the application of the switchable capacitors asillustrated in FIG. 2 b can also be applied in combination with a powerconverter or drive unit DU as applied in the other lighting applicationsaccording to the invention.

In an LED based application, a colour set point is, in general, realisedby operating a plurality of LEDs having a different colour (e.g. a red,a green and a blue LED) each at a specific duty cycle such that, onaverage, the colour set point is obtained. When a substantially constantsupply voltage is available, which is sufficient to power the serialconnection of LED units, a controller or control unit may easilydetermine the required duty cycles of the different LED units. When thesupply voltage is sufficient to power all LED units at the same time,the supply voltage does not pose a limitation to the application of thedifferent duty cycles. When however, e.g. due to the application of adimmer, the supply voltage as provided to the drive unit, is at someinstances, too low to power all LED units at the same time, the controlunit may need to take the available voltage into account whendetermining at which instances which LED units are powered. This isillustrated in the following FIGS. 3 a-3 b, schematically depictingoperating sequences of multiple LED units using a duty cycle modulation,e.g. PWM or the like. Assuming an LED assembly comprising three LEDunits, each comprising one LED of a different colour. In order toprovide, on average, a predetermined colour set point, the LED units areoperated at a different duty cycle, e.g. determined by a control unit.FIG. 3 a schematically depicts when the three LED units are powered,indicated by the required forward voltage Vf of the LED units, as afunction of time t. The operation of the LED units is indicated byrectangular shaped blocks, e.g. block 300, wherein the width of theblock corresponds to the time an LED unit is powered and the height of ablock represents the required forward voltage for providing a current tothe LED unit. As can be seen, in order to operate the LED units at aspecific duty cycle, the LED units are only operated part of each periodT, also referred to as the ‘duty cycle period’. Within the meaning ofthe present invention, the term ‘duty cycle period’ is used to denotethe period over which a required duty cycle is applied. The duty cycleperiods Ti as shown in FIGS. 3 a and 3 b can, in general, be selected inan arbitrary manner and could even be varied during operation.

It has been observed however that, in case a periodic supply voltage isavailable, it may be advantageous to synchronise the switchingoperations of the switch assembly controlling the topology of the LEDassembly with the periodic supply voltage. Such a synchronisation canadvantageously be achieved by selecting the duty cycle period such thata period of the periodic supply voltage divided by the duty cycle periodresults in an integer value. As an alternative, in case the duty cycleperiod is not constant (as e.g. illustrated by periods T2, T3 and T4 asindicated in FIG. 4), a sequence of successive duty cycle periods shouldform a pattern which is synchronised with the period of the supplyvoltage. By synchronising the duty cycle period with the period of thesupply voltage, aliasing effects can be reduced.

By operating the LED units at a specific duty cycle, a specific colourset point can be realised. When the three LED units are powered at thesame time, the sum of the required forward voltages of the LED unitsshould be available. As such, at t=t1, the supply voltage needs to belarger than the sum of the forward voltage of the three LED units (i.e.Vt). At t=t2, only the forward voltage of the LED unit indicated byblock 330 needs to be available. The supply voltage available forpowering the LED units is schematically indicated by the dotted line310. When the intensity of the lighting application needs to be reduced,while maintaining the colour set point, the duty cycles of the LED unitscan e.g. be reduced proportionally. In case a substantially constantsupply voltage is available, the reduced intensity can be realised byoperating the LED units as indicated during period T2.

In case a variable supply voltage is available, powering the LED unitsmay not be possible in the same manner. This is illustrated in FIG. 3 b.The left part of FIG. 3 b (covering periods indicated by T1)substantially corresponds to the left part of FIG. 3 a. During periodT2, a reduced intensity is required, due to the lower available supplyvoltage, while maintaining the same colour set point. (it can be notedthat the human eye is more sensitive to short colour variations than tointensity variations.) Due to a reduction of the supply voltage,indicated by the dotted line 340, the supply voltage available duringperiod T2 is less than the sum of the forward voltages of the three LEDunits. Phrased differently, during period T2, the three LED units cannotbe powered at the same time. It will be clear that solving this bymerely powering only the LED units indicated by blocks 350 and 360 wouldaffect the colour that is generated. In order to operate the lightingapplication at the reduced intensity and maintaining the colour setpoint, the LED units can be powered with the required duty cycle, butsequential powering of the LED units may be required. This isillustrated during period T3 of FIG. 3 b: As can be seen, rather thanpowering the three LED units at the start of period T3, only two unitsare powered, the third unit (indicated by block 370) is powered, at theappropriate duty cycle, when the first unit (indicated by block 380) isno longer powered. As such, the reduced intensity can be realised whilemaintaining the colour set point. As will be clear to the skilledperson, a further reduction of the available supply voltage (e.g.indicated by dotted line 390 during period T4) may result in therequirement that only one LED unit can be powered at the same time. Insuch a situation, the powering of the three LED units can e.g. be asindicated during period T4 of FIG. 3 b. In general, such a sequentialpowering of the different LED units within a certain period rather thanpowering the LED units simultaneously should be feasible since therequired duty cycles of the different units can be reduced, i.e. tomimic the effect of a reduced supply voltage of a conventional lightingapplication on the intensity of the lighting application. In order tomaintain substantially the same colour output, the duty cycles of thedifferent LED units can be reduced proportionally. So, in order torespond to changed dimmer output voltage (i.e. due to a user action on adimmer in order to reduce the light intensity), the control unit mayreduce the duty cycle of the LED units thus realising a reduced lightintensity. Operating the different LED units at reduced duty cyclesenables the powering of the LED units in a more sequential manner (seethe operation during periods T3 and T4) rather than powering the LEDunits at the same time. By operating the LED units in a sequentialmanner, the voltage requirement for powering the LED units is reduced.It may further be noted that an average light intensity over multipleperiods T can e.g. be maintained by compensating for a reduced intensityin periods having a comparatively low supply voltage in periods having acomparatively high supply voltage.

Regarding the selection of the duty cycle period or periods as applied,in a preferred embodiment, the duty cycle period is selected such thatthe division of a first supply voltage period by the duty cycle periodresults in an integer number and the division of a second, differentsupply voltage period by the duty cycle period also results in aninteger number. By doing so, the lighting application can be poweredfrom power supplies with a different frequency while maintaining theadvantages of avoiding aliasing effects. As an example, selecting theduty cycle period equal to 833 microseconds results in both the 50 and60 Hz period being divisible by the duty cycle period.

When the available supply voltage is known in advance, e.g. by samplingand storing a previous waveform of the supply voltage, the control unitof the lighting application according to the invention can determine inadvance an optimal sequence of powering the LED units such that anoptimal use is made of the available voltage while at the same time,taking a required colour set point into account. As an example, FIG. 4illustrates how the powering of three LED units can be performed as afunction of time given a substantially sinusoidal supply voltage for thedrive unit. The dotted lines 450 schematically indicate the requiredvoltage for powering 1, 2, resp. 3 LED units. As can be seen, duringperiod T1, no LED units are powered because the supply voltage(indicated by the curve 400) is too low. During period T2, the availablevoltage is sufficient to power one LED unit. As in FIGS. 3 a and 3 b,the LED units are indicated by rectangular shaped blocks, wherein thewidth of the block corresponds to the time an LED unit is powered andthe height of a block represents the required forward voltage forproviding a current to the LED unit. By sequentially powering the 3 LEDunits, (whereby each LED unit may have a different duty cycle), a userdefined colour set-point can be realised. During period T2, two LEDunits can be powered at the same time, e.g. as indicated. During periodT3, three LED units can be powered at the same time. The control unit(e.g. the control 40 as shown in FIG. 2 a) of the lighting applicationaccording to the first aspect of the invention can be arranged todetermine the appropriate duty cycles of the LED units for operating theLED units during the periods T2, T3 and T4. As such, the voltageavailable for powering the LED units (i.e. the voltage available at theterminals of the power converter that powers the LED units) can beapplied in an efficient manner by adjusting the LED assembly topology insuch manner that the required forward voltage can be provided by the(instantaneously) available supply voltage at the terminals of the powerconverter or drive unit. As will be acknowledged by the skilled person,the principle as described in FIGS. 3 a, 3 b and 4 can be applied to anarbitrary supply voltage available to the power converter.

The lighting application according to the first aspect of the presentinvention can thus be applied to retrofit a number of different lightingapplications that are currently available on the market. Such lightingapplications include, but are not limited to, conventional light bulbsthat are powered from a mains supply or halogen light applications thatrequire a low voltage DC supply. When such applications are providedwith a dimmer circuit, the output voltage of the dimmer circuit which isapplied to the lighting application may vary substantially. The lightingapplication according to the present invention is arranged to acceptsuch varying supply voltage and adjust the topology of the LED assembly(e.g. by short circuiting one or more LED units) based on the availablevoltage.

In an embodiment, the LED driver according to the first aspect of theinvention is arranged to diagnose the available supply voltageautonomously and adjust the control of the LED driver based on thediagnosis. As an example, the waveform analyser as applied in a LEDdriver according to the first aspect of the invention, can be arrangedto determine certain characteristics of the supply voltage (such asaverage value, mean value, top value, frequency (content), etc.) anddetermine, based on the information the appropriate way to control theLED driver and/or LED assembly. Further details on this aspect of thepresent invention are provided below.

According to a second aspect of the invention, an LED driver is providedfor powering an LED assembly. FIG. 5 schematically depicts such an LEDdriver 500, the LED driver can e.g. be arranged to receive an inputvoltage 510 such as a TRIAC dimmer output voltage, e.g. obtained from amains AC voltage 530, for powering the LED driver 500. In order toprovide the dimmer output voltage 510, the TRIAC of the dimmer (notshown) needs to be operable in a conduction state, which, as known tothe skilled person, requires a current (holding current) to flow throughthe terminals of the TRIAC. As such a current may pose an importantdissipation, it may be important to keep this current preferably as lowas possible. The LED driver according to the second aspect of theinvention is arranged to determine, e.g. in an iterative manner, therequired holding current. In addition to the LED driver, FIG. 5schematically depicts an (AC) power source 530 and a dimmer DM 520 (e.g.a phase controlled dimmer such as a TRIAC dimmer). The dimmer outputsignal 510 can, in general, directly be applied to the LED driver. Inthe arrangement as shown however, the dimmer output voltage 510 isrectified by a rectifier REC 540, the rectifier output voltage 550 beingapplied as supply voltage for the LED driver. The LED driver DU as shownin FIG. 5 can e.g. comprise a Buck or Boost converter 560 or the likefor providing power to an LED assembly LED 580. As an example, theconverter 560 may correspond to the converter 50 as depicted in FIG. 1a. The LED driver further comprises a control unit 570 for controllinga.o. the converter 560 of the LED driver. The control unit CU 570 mayequally be applied to control the operating conditions of the LEDassembly. In an embodiment, the LED assembly as powered by the LEDdriver according to the second aspect of the invention can e.g. becontrolled by a switch assembly (not shown) that can e.g. comprise oneor more switches (such as FETs or MOSFETs) to control the currentthrough the one or more LED units of the LED assembly. The LED assemblyas shown in FIG. 5 can e.g. correspond to an LED assembly as applied inan embodiment of the lighting application according to the first aspectof the invention. The LED driver as shown is further arranged todetermine the required current for maintaining a TRIAC of the dimmer ina conductive state. In the embodiment of FIG. 5, this is implemented asfollows: The control unit 570 of LED driver 500 is arranged to graduallydecrease the current as provided to the load, i.e. the LED assembly.This can e.g. be established by controlling the switching operation ofthe switching element of the converter 570. By varying the duty cycle ofsuch a switching element (e.g. the switching element T of converter 50as shown in FIG. 1 a), the current as provided by the dimmer 520 andthus the current as provided through a TRIAC of the dimmer, can bevaried. As an alternative, the control unit 560 can be arranged tocontrol the load as presented by the LED assembly 580 to the LED driver.In general, the variation of the current as drawn by the LED driver fromthe dimmer can be realised by varying the load that is powered by theLED driver or by controlling the LED driver directly. FIG. 6schematically depicts an embodiment of a lighting application 600comprising an LED driver (comprising a power converter or drive unit DU104 and a control unit CU 108) according to the second aspect of theinvention. The power converter 104 preferably comprises a buckregulator, e.g. out of cost, efficiency and size considerations, but mayalso comprise a buck-boost, boost, SEPIC, CUK, etc., or any multiples orany combinations thereof. Such a power convertor generally comprises oneor more switching elements (as e.g. switching element T of converter 50of FIG. 1 a) which can, in the embodiment as shown, be synchronised tothe line phase voltage 99, e.g. by control of the control unit 108. Suchswitching elements can e.g. comprise FETs, bipolar transitors, MOSFETs,etc. The (switched mode) convertor 104 may, in an embodiment, applycapacitors or inductors as storage elements that are switched. Suchembodiments are discussed in more detail below. The power convertor maybe direct off-line (not galvanically isolated) or galvanically isolatedby e.g. a flyback transformer and accompanying switching element.

In the embodiment as shown, the variation of the current requirement ofthe LED driver (i.e. the current provided to the converter 104) isenabled by a variable load 111. As a variable load, the LED driver inthe embodiment as shown, is arranged to provide a current to a resistorwhich is arranged in series with the LED assembly. The load as providedby the resistor can be varied by operating a switch (e.g. a FET orMOSFET) that is provided to short circuit the resistor. The switch canbe operated at a comparatively high frequency at a variable duty cycle.As such, the resistor can represent a variable load which cansubstantially continuously be varied. By varying the load that ispowered by the LED driver, the current that is e.g. provided by a TRIACdimmer to the LED driver will vary as well. As such, by varying the dutycycle at which the resistor is operated, the current provided by theTRIAC dimmer can be gradually reduced. In accordance with an embodimentof the LED driver according to the second aspect of the invention, thecontrol unit 108 of the LED driver can be arranged to control thevariable load (represented by the switchable resistor 111) in order toreduce the supply current (i.e. the current as supplied by the TRIACdimmer to the LED driver) and to measure a value of the input voltage(e.g. at an input terminal of the LED driver) in relation to the supplycurrent. When the load is varied to such extend that the voltage at theterminals of the LED driver drops to zero, the load as presented by theLED driver is too small to maintain the TRIAC in a conductive state.Based on this, the control unit 108 can determine a minimum loadrequirement (or minimum supply current) for maintaining the TRIAC in aconductive state. Based on this information, the control unit 108 can bearranged to ensure that this minimum current is required by the LEDdriver's converter 104 during the time a voltage is required at theterminals of the LED driver. By doing so, the LED driver according tothe second aspect of the present invention, enables a TRIAC dimmer to beoperated at a minimum holding current as required by the TRIAC dimmer,the minimum current being based on the actual operating conditionsrather than being set to a fixed holding current. By doing so, animportant improvement of the efficiency of a lighting applicationcomprising the LED driver can be realised: The minimum holding currentof a TRIAC may vary substantially depending on the operating temperatureof the TRIAC. At −40 C, a minimum holding current of approx. 30-50 mAmay be required, while at +25 C an average TRIAC only requires 5-10 mA.When the LED driver according to the second aspect of the presentinvention is not applied, the minimum load requirement of the LEDassembly may need to be set such that a comparatively high current (e.g.30 mA) is provided by the TRIAC, at all times, in order to ensure theconductive state while, due to the operating conditions, a much smallercurrent would be sufficient (e.g. 5 mA or less) to maintain the TRIAC ina conductive state. As will be acknowledged by the skilled person,operating the TRIAC dimmer at the actual minimum holding current (i.e.based on the operating conditions) can provide an important efficiencyimprovement of the lighting application applying the LED driver. As, dueto varying operating conditions, the minimal holding current may alsoincrease over time, it may be advantageous to operate the LED driver atan elevated minimum current (e.g. 10% above the minimum supply currentas determined by the control unit). In such an embodiment, the controlunit can thus be arranged to adjust the load of the LED driver (i.e. theLED assembly and/or the switchable resistor) to the control unit can bearranged to ensure that the minimum current as provided to the LEDdriver during the time a voltage is required at the terminals of the LEDdriver is above the minimum holding current of the TRIAC.

By providing a variable load in series with the LED assembly andcontrolling the load in a similar manner as the duty cycle of an LEDunit of the LED assembly is controlled (e.g. by providing a switch (e.g.a FET or MOSFET) in parallel, as illustrated in FIG. 1 a), use is madeof the already available topology of the LED assembly and switchingassembly. In addition, the embodiment as illustrated provides theadvantage that the load can be varied with a comparatively highresolution, e.g. the same resolution as can be applied to the duty cycleof the LED unit. As such, the minimal holding current can be determinedwith a high accuracy.

The lighting application 600 as schematically depicted in FIG. 6 mayfurther comprise the following components which can e.g. correspond tosimilarly numbered components of the lighting application as shown inFIG. 2 a. As such, the lighting application can e.g. comprise, whenrequired, an EMI Filter EMI 100 as discussed in more detail above. Afurther optional element is the power factor correction (PFC) elementPFC 101 that can e.g. compensate for current distortions (e.g. due tocapacitive and inductive loading versus purely resistance) to meetregulatory requirements on PFC. When the driver properties aresufficient to meet these requirements no PFC parts are added. For lowpower applications from 1 to about 30 W it is, in general, sufficient toapply a solution comprising diodes and capacitors for compensation. Forhigher power levels, an active mode for PFC can e.g. be used, oftenconsisting of an additional boost mode power convertor, buck orbuck-boost convertors are other options for active mode PFC as well.

As already discussed above, the input voltage 99 of the lightingapplication can e.g. correspond to a TRIAC dimmer output voltage.Optionally, a rectifier element REC 102 can be applied to rectify theincoming waveform thereby generating a pulsed DC waveform that can beapplied to supply the power convertor 104. In its simplest form itconsists of a single diode which then leads to a single phase outputwith a large off-period. The preferred embodiment is a diode bridgeconsisting of 4 diodes that use both AC phases and lead to a fullyrectified output. Due to the minimum forward voltage drops the diodescause some current and voltage distortion and also account for somedissipation. Most of the dissipation can be removed by using low voltagedrop switching elements (e.g. FETs) instead of the diodes.As discussed above, the lighting application can be provided with aninput filter and/or switchable buffer SB1 103 having the purpose ofsupplying the power convertor 104 when the voltage at the rectifier 102output is below a minimum level suitable for the power convertor 104. Asan example, the input filter can e.g. be a fixed input filter bufferdirectly connected to the rectified voltage (i.e. an output voltage ofthe rectifier 102. As an example of the switchable buffer, such a buffercan comprise a buffer element that can be temporarily connected to therectified voltage in order to the buffer and, at a later time, connectthe buffer element to the input of the power converter in order todischarge the buffer in order to supply power to the power convertorwhen the rectified voltage has fallen too low. The advantage of(dis-)connecting the buffer is to save the stored high voltage in thebuffer for optimal use at the time it is needed. The moments for(dis-)connecting can be done autonomously in this element by alwaysaccepting charging and saving discharging only when the input voltagedrops below the minimum level of the power convertor. A preferredembodiment however would give control to the controller (108) which cansynchronise the (dis)-connection time-frames with the line phase whichgives a more guaranteed buffer charging of each power cycle independentfrom momentary distortions of the line voltage. The buffer (or storage)element can e.g. comprise a capacitor but can also take the form of aninductor. The embodiment of the lighting application 600 as shown inFIG. 6 may further, optionally, comprise an output filter and switchedbuffer SB2 105 as described in more detail in FIG. 2 a. Further detailson a switchable buffer as e.g. applied in elements 103 or 105 of FIG. 6are provided below.

Determining the actual minimal holding current can be done periodicallyin accordance with the period of the AC supply voltage of the TRIACdimmer. It can be done each period, or less frequent, e.g. every 10 ormore periods. The process of determining the minimal holding current asperformed by the control unit of the LED driver according to the secondaspect of the invention can preferably start from a minimum holdingcurrent as applied during a previous period. The lighting application600 as shown in FIG. 6 further comprises a phase detector 107. Such aphase detector, can e.g., preferably over multiple cycles, determine asynchronisation of the controller to the line frequency. This can forexample be done by (software) PLL locking to compensate for cycle-localline voltage distortions. The control unit 108 may then base all itscontrol moments in time on the line synchronisation. It can be notedthat a waveform analyser as e.g. applied in a lighting applicationaccording to the first aspect of the invention can be applied as a phasedetector 107.

In an embodiment, the process of determining the minimal holding currentto be provided to the LED driver is done during a part of the inputvoltage period when the LED units are not emitting light. This can e.g.be a trailing end of the periodic signal as provided by the TRIAC dimmerto the LED driver. This is schematically indicated in FIG. 7 a. FIG. 7 aschematically depicts an output voltage V of a TRIAC dimmer circuithaving a period T, as a function of time t. Due to phase control of theTRIAC, only part (T1) of an e.g. sinusoidal input voltage is availableat the output terminals of the TRIAC dimmer. In principle, the outputvoltage of the TRIAC dimmer (optionally after rectification by arectifier) can be applied for powering an LED assembly during the entirepart T1 of the period T, i.e. during the time when the TRIAC isconducting. As will be acknowledged by the skilled person, in order topower an LED or LED unit, a minimum voltage is required. As will beclear, such a minimum voltage requirement may depend on the actualtopology of the LED assembly; when the LED assembly comprises multipleLEDs connected in series, the minimum voltage requirement for poweringthe multiple LEDs substantially corresponds to the sum of the forwardvoltage of the individual LEDs. In FIG. 7 a, such a minimum voltage isindicated by the dotted line 700. As a consequence, part T2 of thevoltage is insufficient to power an LED unit. Part T2 of the outputvoltage period T may however be applied to determine the minimal holdingcurrent. During this part of the period T, the control unit can (asdiscussed above) gradually reduce the supply current to the LED drivere.g. by varying the load e.g. represented by a switchable resistor untila voltage drop to zero is observed and thus the minimal holding currentis found. As the analysis is performed during part of the period whereinthe LED or LED units of the LED assembly are not emitting light, a loadflicker due to the TRIAC losing its conductive state will not beobserved by the user. In general, the entire part T1 of the period Tcould be subdivided into a part T3 dedicated to be applied to power theLED assembly and a part T4 dedicated to determine the minimal holdingcurrent. The part dedicated to determine the minimal holding current ispreferably selected to be the trailing part of T1 (i.e. trailing to thepart applied to power the LED assembly) as determining the minimalholding current may result in the TRIAC losing its conductive state,which could e.g. be observer as load flicker.

The minimum holding current as determined, can subsequently be appliedduring a next period of the dimmer output voltage to set the requiredLED assembly load and, if required, a variable load.

In case only part of the available voltage part T1 is applied to powerthe LED assembly, a further efficiency improvement can be obtained asfollows: when the process of determining the minimal holding currentduring part T4 of the voltage T1 is only applied during one period every5 or 10 periods, the load current as provided by the dimmer circuit tothe LED driver may be reduced to zero during the other periods. As such,the minimum holding current is only supported during that part of thevoltage period T1 that is used to power the LED assembly, i.e. part T3.The dissipation associated with maintaining the minimum holding currentduring part T4 of the voltage period T1 may thus be reducedsignificantly.

To illustrate this, FIG. 7 b schematically depicts how a minimal holdingcurrent is supported in a selected interval for power efficiencyreasons. In FIG. 7 b, graph a depicts a rectified idealized voltageinput as e.g. provided by a TRIAC dimmer. Graph b shows the intervalswhere the holding current needs to be drawn in order for the TRIAC tostay on when a voltage V as shown in graph a is available (i.e. notequal to zero). Graph c of FIG. 7 b schematically depicts the availablevoltage (thick line) in case a buffer capacitor is applied (more detailsof such an arrangement are provided below). It is assumed that thebuffer capacitor is used to supply the load as from instance 703. Assuch, it can be noted that current is actually only drawn from thedimmer between instances 702 and 703. As such, the TRIAC may abort itsconducting phase too early which can be considered improper behaviourfor stable light output. In graph d of FIG. 7 b, curve 705 shows achosen (e.g. by the control unit 108 as shown in FIG. 6) segment of timewhere the holding current will be supported. In graph e of FIG. 7 b, theresulting waveform from the TRIAC input voltage can be seen. As a resultof the controlled support of the minimal holding current, between 707and 709 the holding current is guaranteed. By the methods mentionedabove it is still possible to determine the required dimming level byregularly testing a full or half period of the supply voltage for theappropriate TRIAC dimming level setting.

The LED driver according to the second aspect of the invention can e.g.be applied in a lighting application comprising an LED assembly and aswitch assembly.

In an embodiment, the LED driver according to the second aspect of theinvention is provide with an input buffer such as a capacitance. Such aninput capacitance can be applied as a buffer for providing a supplyvoltage to the LED driver when the supply voltage is comparatively low.Such a capacitance may equally serve as a filtering element.

When an LED driver such as an LED driver according to the second aspectof the invention, is powered by a TRIAC dimmer output voltage, the LEDdriver should adjust the brightness of the LED assembly that is poweredwhen the dimmer is operated, thereby e.g. mimicking the dimming of alight bulb that is powered by a dimmer circuit. In order to adjust thebrightness, the LED or LED units of the LED assembly can e.g. beoperated at a different duty cycle. In order to assess the dimmer level,various options exist:

As a first example, the average dimmer output voltage can be determinedand provided as an input signal to the control unit. In order to obtainsuch an input signal, the dimmer output voltage can be rectified andfiltered such that a DC signal is obtained.

As a second example, the LED driver can be arranged to analyse thedimmer output voltage and determine a brightness set point based on theanalysis. In an embodiment, the LED driver is provided with a phaseanalyser for analysing the dimmer output voltage. The phase analyser cane.g. determine, based on the dimmer output voltage or a signalrepresenting the dimmer output voltage, the zero crossings of the dimmeroutput voltage. As such, a phase analyser may equally determine thephase angle describing the phase cut made by the dimmer circuit. Basedon this, the control unit can determine a set point for the brightness.

It can further be noted that the waveform analyses as e.g. applied inthe lighting application according the first aspect of the invention mayequally be applied to facilitate the determination of a brightnessset-point for the control unit of the LED driver.

As a third example, in case a switching converter such as a Buck orBoost converter is applied for powering the LED assembly, monitoring theduty cycle of the switching converter can be used to determine therequired dimming level. This can be understood as follows with referenceto FIG. 1 a: When a switched converter as the Buck converter 50 shown inFigure la is applied to power an LED assembly, the converter 50 is, ingeneral, controlled to provide a substantially constant output currentto the LED assembly. In order to maintain such a constant outputcurrent, a switching element of the converter, e.g. switch T ofconverter 50 in FIG. 1 a, will operate at a certain duty cycle. In casethe input voltage V of the converter would change, this change wouldaffect the duty cycle of the switching element. A larger input voltage Vwould require the switching element T to operate at a smaller duty cyclein order to maintain the output current I at the same level. Note thatthe output current I (or load current) can be determined from thevoltage drop over the sensing resistor Rs. This mechanism can be appliedto adjust the brightness of an LED assembly in the following way.Assuming that an increase of the input voltage would lead to theswitching element T operating at a smaller duty cycle. By setting thebrightness set point higher (and/or change the topology of the LEDassembly), the power drawn from the regulator/power converter 50 isincreased causing the duty cycle of the switching element to increaseagain. So by changing the set point of the brightness (and thus thedimming level), the duty cycle of the converter switching element iskept substantially constant, and the dimming level will substantiallyfollow the incoming average voltage level and thus the TRIAC dimmersetting.

In such an embodiment, no additional hardware such as an ADC (analogueto digital converter) for providing a signal to the control unitrepresenting the input voltage.

As a fourth example, which will be explained in more detail below, therequired dimming level can be derived from a voltage available over abuffer or switchable buffer as e.g. applied in an LED driver accordingto a third aspect of the invention,

It is worth noting that the assessment of the appropriate dimming levelas described by the above examples, may be applied in any of the driveunits or converters as applied in the present invention.

As already discussed above, in order to remain in a conductive state, acurrent that is equal or larger than the holding current (depending onoperating conditions such as temperature) needs to flow through theterminals of the TRIAC.

LED drivers as generally applied to power LED assemblies comprise aconverter for providing a substantially continuous DC current to an LEDassembly. Such a converter can e.g. be a Buck converter as schematicallydepicted in FIG. 1 a. Depending on the state of the switching element Tof the converter, the current as provided to the LED assembly isprovided via the diode D or via the switching element T. As such, theactual current that is drawn by the LED driver e.g. from a dimmercircuit is a pulsed current. As an examples, the switching element T ofthe converter can operate at a 500 kHz frequency.Due to input filtering before the LED driver, e.g. by an input filter asdiscussed above or the application of an EMI filter, the current asprovided by the TRIAC dimmer is a substantially continuous current (whenthe TRIAC is conducting) rather than a pulsed, e.g. at 500 kHz, current.It can be noted that, in practice, an actual input filter is notrequired to ensure that the TRIAC dimmer provides a substantiallycontinuous current rather than a pulsed current due to the wiring thatis available between the dimmer and the light source, i.e. the LEDassembly.

As explained above, the LED driver according to the second aspect of theinvention enables an energy efficient application of a TRIAC dimmer inthat the LED driver can determine the minimal required holding currentand operate the LED driver in such way that, when a supply voltage isrequired, the minimal holding current is drawn by the LED driver.

The LED driver according to the second aspect of the invention isparticularly suited to be applied in a lighting application havingmultiple LED assemblies, each LED assembly being powered by an LEDdriver, the multiple LED drivers being powered by a common TRIAC dimmer.In such an arrangement, the required minimal holding current can e.g. bedrawn by only one of the LED drivers or as the sum of a minimal currentdrawn by two or more of the LED drivers.

As explained above, an LED driver can e.g. be provided with a so-calledinput buffer (e.g. a capacitance) which can be applied for providing asupply voltage to the LED driver when the supply voltage iscomparatively low. Such a capacitance may equally serve as a filteringelement. Such an input buffer can be significantly reduced when aswitching element is provided, e.g. controlled by a control unit of theLED driver, for connecting and disconnecting the buffer to the supplyvoltage. Therefore, according to a third aspect, the present inventionprovides an LED driver for powering an LED assembly comprising at leastone LED, the LED driver comprising a converter for converting a periodicinput voltage to a supply current for powering the LED assembly, the LEDdriver comprising a converter having input terminals for receiving theperiodic input voltage, the LED driver further comprising a control unitand an input buffer connectable to the terminals by a switching element,the control unit further being arranged to control the switching elementto connect and disconnect the input buffer to the terminals.

The LED driver according to the third aspect of the invention can bee.g. be applied in an application as e.g. shown in FIG. 2 a, 5 or 6.

When an LED driver is powered by a periodic input voltage (e.g. arectified AC voltage or a dimmer output voltage), it may occur that theinput voltage is insufficient to power a LED assembly. Such a situationis illustrated in FIG. 8.

In FIG. 8, a rectified sinewave 601 as e.g. obtained as output voltageof a rectifier (e.g. rectifier 102 as shown in FIG. 2 a, 5 or 6) isshown. During a certain interval, the output voltage may drop below aminimum supply level 602, e.g. required by a power converter (e.g. powerconverter 104 as shown in FIG. 2 a, 5 or 6). The resulting waveform 601is the supply waveform as received by a power converter (e.g. converter104 of FIG. 2 a, 5 or 6) and is shown as a bold line. The converter mayprovide a current through an LED assembly (e.g. assembly 110 of FIG. 2a, 5 or 6) at a certain duty-cycle as e.g. depicted at 603 and 604. Itcan be noted that, to obtain a varying light output, the duty cycle andfrequency thereof may change over time. A controller or control unit(e.g. control unit 108 of FIG. 2 a, 5 or 6) can e.g. control the powerconverter and/or a switch assembly such as switch assembly 106 of FIG. 2a, 5 or 6 to provide the required current (e.g. amplitude, duty cycle orfrequency). As illustrated in FIG. 8, when the timing of switching theLED current on and off is not synchronized to the frequency of thesupply voltage (e.g. a mains supply voltage), a certain amount of lightoutput can be lost (indicated by the grey surfaces at 606). The size ofthe grey surfaces may depend on a number of factors. Under theassumption that the switching of the LED current is not synchronised tothe supply voltage, the control unit is unaware of the interval when thesupply voltage is too low. Further, there may be uncertainty withrespect to the timing when the voltage drops to zero (608) due tovariation in the supply voltage at moment 608 as well as an uncertaintyin the level 607 where the power converter and control unit areineffective in providing a current to the LED assembly. Such uncertaintymay even be increased by disturbances 609 which can be present on thesupply voltage and are usually only partially filtered. All theseeffects may cause an undesirable fluctuation of the light output ofseveral percent, which can be easily observed by humans. In general,such uncertainty may have a base frequency which is double (in case of afull bridge rectified voltage) the supply voltage frequency, e.g. 50 or60 Hz in case of a mains supply.

To substantially remove these effects, the LED driver according to thethird aspect of the invention can be synchronised to the frequency ofthe periodic supply voltage and/or be provided with a switchable bufferfor, at least partly, bridging an insufficient voltage supply. Bysynchronising the power supply to a LED assembly with the periodicsupply voltage, a window 610 could e.g. be created in which no LEDcurrent is allowed to flow. By doing so, an uncontrolled light outputcan be avoided.

The application of a switchable buffer (embodiments of which arediscussed in more detail below) is illustrated in the following FIG. 9.

In FIG. 9, a full bridge rectified waveform 401 (thin line) is shown ase.g. provided by a rectifier such as the rectifier 102 of FIG. 2 a, 5 or6, in case the rectifier is not loaded. The dotted line 402 indicates aminimum level of supply voltage as required by a load of the rectifier.Such a load can e.g. comprise any combination of components 103 to 110as depicted in FIG. 2, 5 or 6. In case the voltage waveform 401 would besupplied to the load, the load would not be powered properly betweeninstances 405 and 406 since the available voltage 401 is lower than therequired level 402.

By applying a switchable buffer, it is feasible to deliver a voltagehigher than the 404 level between instances 405 and 406 withoutsignificantly compromising the power factor. To that end, a switchablebuffer such as a capacitor can be connected to the available voltage 401as long as voltage over the capacitor is lower than the availablevoltage 401 (f.e. through a diode). From instances 403 through 404, aninitial charging of the capacitor is shown, for example after an initialpower-up. As soon as the 401 voltage diminishes at instance 404, thecapacitor can be disconnected from voltage 401 (e.g. by operating aswitch connected in series to the capacitor, see further on) and thevoltage over the capacitor can remain at level 409. At instance 405 thecapacitor can be re-connected to voltage 401 thereby raising the voltageprovided to the load (e.g. any combination of components 103 to 110 asdepicted in FIG. 2, 5 or 6) to the capacitor voltage at level 409 as thediodes of the rectifier (e.g. rectifier 102 of FIG. 2, 5 or 6) willreverse (indicated by the waveform in bold). Due to the load drawingcurrent from its supply voltage (see 413), the capacitor voltage willdrop accordingly as indicated by the part of the waveform at 410. Atinstance 406, the capacitor can be disconnected again and at instance407 the charge/discharge scheme can be repeated. In general, the buffercan be connected to the voltage 401 when the voltage exceeds the voltageover the capacitor. The capacitor can e.g. be disconnected from thevoltage 401 as soon as the voltage 401 starts diminishing. Thedischarging of the capacitor can e.g. be triggered by the supply voltagedropping below a certain level, e.g. the minimum voltage for poweringthe load. In this way, while the supply voltage towards the load is keptabove the minimum level, the amount of energy stored in the capacitor isused as little as possible between instances 405 and 406 and repetitionsthereof, causing the capacitor to be recharged only minimally frominstance 405 until instance 406. A lower charge current may results,thus compromising the power factor less. Further improvement is possibleby distributing the load in time so that the least current is drawn atthe times the capacitor supplies it. This can e.g. be established bye.g. lowering the current as provided to the load (i.e. providing acurrent with a smaller amplitude) or by changing (reducing) the dutycycle of the current when the supply voltage originating from therectifier is comparatively low. The latter solution is illustrated inFIG. 9 as can be seen from the load difference between 411 and 412 andthe resulting capacitor currents at 413, 414 versus 415 and 416.

By redistributing the load in accordance with the available voltage, asillustrated, the energy drawn from the capacitor can be minimised thusoptimising the power factor.

A first example of how such a switchable buffer can be realised isschematically depicted in FIG. 10. In FIG. 10, an AC supply voltage isprovided at terminals 801 and 802. A double rectifier (Graetz-bridge)comprising diodes 803, 804, 810 and 811 may deliver a waveformcomprising equal half sine-waves at a frequency that is double that ofthe AC input. This waveform can be influenced by the load to therectifier bridge. Typically this load comprises a permanently connectedcapacitor and the actual load, e.g. a power converter of a LED driver.In the embodiment as shown, the load comprises a switchable capacitor805 (that can be connected and disconnected to the rectified voltage 812through a switch 806 (e.g. a FET or a MOSFET) and a load 808 controlledby a control unit 809. The load can e.g. be any of the lightingapplications as described above. By providing a switchable capacitorrather than a fixed capacitor, the following objectives may be pursued:By providing a switchable capacitor, a comparatively small capacitancemay be sufficient to bridge a gap (in time) where the supply voltage isinsufficient. As such, non-electrolytic types of capacitors can beapplied, which, in general, have a longer life-time compared toelectrolytic capacitors. A further objective that can be realised is toimprove the power factor by diminishing current peaks due to charging ofbuffer capacitors.

The embodiment of FIG. 10 further shows an optional filter capacitor 820which can e.g. be applied for EMI reduction. In an embodiment, thecapacitance of the filter capacitance 820 can be comparatively small(e.g. a few microfarad) compared to the switchable capacitor 805. Incase a filter capacitor 820 is applied, it may be advantageous toprovide a switch 816 which can e.g. be opened when capacitor 805 isdischarging thereby avoiding the capacitor 805 to provide energy to thefilter capacitor rather than to the load 808. By doing so, apart from anefficiency improvement, an EMI improvement and audible noise improvementcan be realised as well. Preferably, the switch 816 is provided betweennodes E and F of the ground conductor 814, rather than between nodes Cand D of the live wire or conductor having the supply voltage 812.

A second embodiment of a switchable buffer is schematically depicted inFIG. 11. The terminals A (901) and B (902) as shown can e.g. correspondto the live and neutral wire respectively of a mains network at f.e.230V_(AC) or 120V_(AC), or to any AC source. Diodes 903, 904, 910 and911 are arranged to form a Graetz bridge for double sided rectificationof the AC waveform. To explain the operation of the arrangement asshown, it is assumed that the voltage at A is higher than the voltage atB. The same description can be applied for the case where B is higher involtage than A by using B where A is written and A where B is written.

As depicted, a load L (908) is connected directly to the doublyrectified voltage. Assuming that the load requires a minimum voltageVmin (e.g. corresponding to voltage level 402 in FIG. 9) to operate, theload may sometimes not function properly e.g. when the voltage 912 dropsbelow Vmin. It is often seen that a capacitor is placed across the loadto buffer the energy to supply the load during the times that the ACvoltage supplied to A and B has too low an amplitude. By applying thetopology as schematically depicted in FIG. 11, the voltage across thecapacitor can be summed to the voltage on the A terminal during theperiod in time that the load would otherwise be supplied with a voltagelower than Vmin. To achieve this the capacitor 905 is made switchable.In a first state, the capacitor can be connected to terminal B usingswitch 907 to charge it until it has substantially the maximum amplitudeof the applied AC voltage between A and B across its terminals. Thebottom terminal of capacitor 905 can be disconnected by opening switch907 when the maximum charge voltage has been reached. As a result, thevoltage across the capacitor can substantially stay at the same value,which is substantially equal to the maximum amplitude of the supplied ACvoltage at terminals A and B. Subsequently, the voltage across the loadmay diminish from a maximum amplitude to 0. Before reaching 0, at Vmin,the topology as shown enables the capacitor's voltage to be added to thevoltage at terminal A by closing switch 906. As a result, the voltageacross the load may rise to the sum of the voltage at A and the voltageacross the capacitor. Diode 903 will reverse. When the load's currentcan thus be delivered by the capacitor, the AC voltage may reverse sothat the voltage at A will be lower than that on B. When the voltagedifference between B and A is higher than the voltage on the capacitor,diode 904 may become conductive and the capacitor may charge again. Incase the voltage at A is smaller than the voltage at B, the abovedescription can now be repeated reading B where A is written and readingA where B is written. As will be acknowledged by the skilled person,other topologies of switchable capacitors enabling the voltage availableover a charged capacitor to be added to the supply voltage (or rectifiedsupply voltage) can be devised as well. The embodiment of FIG. 11 ismerely serving to illustrate the principle.

It can be noted that more advanced schemes of switching the capacitor(in general the buffer) can be used. Such schemes can e.g. apply one ofthe following approaches: The first approach is to use knowledge aboutthe form of the sine wave combined with the value of its period (f.e. 20ms). This can be called a time based approach. Using such an approach,the switches can be operated on a certain moment in time relative to thestarting of a new half-period. The second approach is to monitor thevoltage levels and behaviour at certain nodes in the topology. Thefollowing situations can be distinguished. In situation 1, the voltagewaveforms at terminal A (901) is measured f.e. using an ADC(analogue/digital converter). In situation 2 the voltage is measured at912. In situation 3 the voltage Vm (915) is measured.

For low voltages like for example 12 VAC, the peak value of therectified voltage would be approximately 17V. Subtracting 2 times adiode forward voltage (for example when A>B the diodes 903 and 911 wouldconduct), the peak value of the supply voltage as seen by the load wouldbe approximately 15V or even lower. To supply a typical 4 LED RGBWserial topology, a voltage of between 11.5V and 16V is required,depending on the LED type used. Considering also the voltage loss acrossthe converter or drive unit (e.g. a Buck converter), the final voltageacross the LEDs would be even lower. In order to mitigate the voltageloss across the diodes, the following embodiment can be applied:

The diodes 903 and 904, and the diodes 910 and 911 in FIG. 11 may causea lower supply voltage to the load 908 because of their forward voltagedrop. A method of avoiding this is to replace the diodes by switchesthat are controlled by a microcontroller (in general, a control unit) μCto close and open substantially at the same times a diode would switchfrom conducting state to non-conducting state and vice versa.

In an embodiment of the invention (schematically depicted in FIG. 12)such a switch can e.g. be a FET with a built-in diode as schematicallyindicated by 503, 504, 505, 506, 507, 511 and 512. In this way, a FETcan be controlled by the mains voltage itself. When for example a FET isnot conducting in a certain half-period and the AC supply voltagereverses, the diode in the corresponding FET will start conducting sothat the flow of the current is guaranteed. By connecting the gate ofthat FET to the opposite AC connection (when the FET's source isconnected to A, the gate would be connected to B and vice versa), theFET would start conducting when the difference between B and A wouldhave grown to approximately 2.5V. From that moment on, the voltage losswould be only in the order of a few tenths of a volt. For higher ACvoltages, a protection circuit could be placed in the gate control paththat would limit the Vgs to a value below the maximum allowable voltage.

Note that the circuit comprising FETs 511 and 512 can function for allkinds of input voltage, AC as well as DC.

When FETs instead of diodes are used, the switching to the appropriatephase resulting in the rectified voltage, does not occur automatically.In order to attain substantially the same rectified output voltage(apart from the voltage drop over the diodes) as when a full bridgerectifier is used, the following rules should be implemented (e.g. inthe controller or control unit controlling the FETs):

In order to ensure proper operation, the following rules for controllingthe FETs should be obeyed:

Rule1: The FETs 503.1 and 503.2 may not conduct simultaneously.

Rule2: The FETs 511 and 512 may not conduct simultaneously.

Rule3: Gates 503.1 and 503.2 may not put the corresponding FET inconduction mode when the capacitor is used to provide the supplyvoltage, i.e. when in FIG. 12 the FETs 504/505 or FETs 506/507 areconducting).

In order to realise these rules, the gates of FETs 503.1/503.2/511 and512 should be controllable rather than being connected to the ACterminals A and B.

Also the FETs 503.1 and 503.2 could be chosen to not contain a diode. Inthat case, the gate control signal must obey some rules which can beimplemented in the micro-controller.

Also the FETs 503 and 504 could be chosen to be N-FETs instead ofP-FETs. In the latter case the control is more difficult as the gatesneed a voltage higher than the maximum voltage available anywhere in thecircuit. Some kind of boost circuit known in literature could be used.This would form a cost advantage.

Note that, in case the switchable buffer as shown in FIG. 10, is used,it can equally be implemented by FETs, replacing diodes 803, 804, 810and 811 by FETs 503.1, 503.2, 511 and 512. In this case, the gates ofFETs 503.1, 503.2, 511 and 512 can be connected as shown in FIG. 11.

FIG. 12 further schematically indicates a more detailed implementationfor the switches 906 and 907 as indicated in FIG. 11. As a switch, twoFETs (e.g. 504 and 505 for connecting to A and 506 and 507 forconnecting to B) are connected back to back and controlled with 1 mutualgate connection (509 and 510 respectively). The gates can be controlledfrom the micro-controller 514 but also by hardware-only solutions. Notethat the 906 and 907 switches of FIG. 11 could also be replaced by 1 FETper switch when FETs without internal diodes are used. It can however benoted that FETs without internal diodes are rarely used.

It is worth noting that, in order to obtain an implementation of acontrol unit (or controller) as e.g. shown in FIG. 1 a, 2 a, 5 or 6, usecan be made of a finite state machine or similar control concept knownto those skilled in the art, in order for the control unit to respond toone or more varying conditions. Referring to FIG. 2 a, the control unitcan e.g. be arranged to respond to (changes in) signals of the(external) power supply of a LED driver, e.g., measured at terminals 99or in between components (e.g. components 102 and 103) or measuredinside one of the components. Such signals can e.g. be provided to acontrol unit (e.g. via an analogue to digital conversion in case of adigital controller) and can be interpreted by the controller in order toassess if certain conditions are fulfilled. Such conditions, e.g.measured on the supply voltage or on the supply voltage betweencomponents 103 and 104 can e.g. be “top of voltage reached”, “zerocrossing detected”, “waveform trend is positive”, “time passed sincetop, is larger than X milliseconds”, “voltage is above or below acertain threshold”, etc. It will be clear to the skilled person thatother conditions as obtained from other components (such as the LEDassembly or the waveform or phase analyzer can also be used as input forthe control unit in order for the control unit to determine if certainconditions are met or not.

The present invention encompasses, as explained above, various aspects.The present invention e.g. discloses various embodiments of LED driversthat can e.g. be applied to retrofit existing lighting applications toLED lighting applications.

It will be clear the skilled person that the functionality as providedby the different LED drivers and lighting applications according to theinvention can be combined. As an example, the lighting applicationaccording to the first aspect of the invention, which enables thetopology of an LED assembly to be changed in accordance with a supplyvoltage may be arranged to include the functionality of an LED driveraccording to the second aspect of the invention (i.e. determining aminimal holding current) or may be expanded with a switchable buffer ase.g. provided in an LED driver according to the third aspect of theinvention.

In order to facilitate a retrofitting, it is worth noting that in anembodiment, an LED driver according to the present invention candetermine, by applying a diagnostics program, which form of supplyvoltage is available at the LED driver terminals when the LED driver isconnected to the power supply providing the supply voltage. As known bythe skilled person, various ways of powering lighting applications areapplied on the market at present. The following list is merely intendedto be illustrative rather than being limited:

A power supply for a lighting application can e.g. provide one of thefollowing voltage forms:

-   -   x V AC (at different frequencies such as 50/60/400 or 480 Hz)    -   x V DC    -   x V as provided by an electronic transformer    -   . . .        When a LED driver according to the present invention is        connected to such a voltage source, a waveform analyser or phase        analyser may (when applied) determine, based on the voltage        available (e.g. based on the minimum/maximum/average        voltage/frequency spectrum of the voltage) determine the nature        of the supply voltage. Depending on the outcome of the analysis        or diagnosis, an optimal switching for a switchable buffer can        be applied.        It can further be noted that such diagnoses of the supply        voltage can be applied as an initialization or can be applied        substantially continuously, in order to adjust a control scheme        of e.g. a switchable buffer depending on the available supply        voltage.

It should further be mentioned that the embodiments of the LED driversand lighting applications as described are mere illustrations of thevarious aspects of the invention, the invention only being limited bythe scope of the claims as set forth.

The invention claimed is:
 1. An LED driver for powering an LED assemblycomprising at least one LED, the LED driver comprising a converter forconverting a periodic input voltage to a supply current for powering theLED assembly, the LED driver further comprising a control unitconfigured to determine a minimal holding current by, in use, graduallyreducing the supply current until a value of the input voltage of theconverter substantially reduces to zero and the control unit is furtherconfigured to subsequently control the converter to operate at a supplycurrent at least equal to the minimal holding current.
 2. The LED driveraccording to claim 1 wherein the control unit is configured to determinethe minimal holding current by performing, in use, the followingfunctions:
 1. operating the converter to reduce the supply current, 2.measuring a value of the input voltage at the reduced supply current,and
 3. repeating steps 1 and 2 until the supply voltage substantiallyreduces to zero.
 3. The LED driver according to claim 1 wherein thecontrol unit comprises an output terminal for outputting a controlsignal to the converter for gradually reducing the supply current and aninput terminal for receiving a signal representing the value of theinput voltage.
 4. The LED driver according to claim 1 wherein a supplyvoltage comprises a TRIAC dimmer output voltage.
 5. The LED driveraccording to claim 1 wherein the LED driver further comprises inputterminals for receiving a supply voltage.
 6. The LED driver according toclaim 5 further comprising an input capacitance arranged between theinput terminals.
 7. The LED driver according to claim 1 wherein thecontrol unit is arranged to determine a dimming level from a supplyvoltage.
 8. The LED driver according to claim 1 wherein the minimalholding current is maintained during at least an entire period of theinput voltage in order to determine a dimming level.
 9. The LED driveraccording to claim 1 wherein the control unit is arranged to determinethe minimal holding current at least every 10 periods of the inputvoltage.
 10. The LED driver according to claim 1 wherein a trailing endof a period of the periodic input voltage is applied to determine theminimal holding current.
 11. A lighting application comprising the LEDdriver according to claim 1, the lighting application further comprisingan LED assembly comprising at least one LED, the lighting applicationfurther comprising a variable load, in use controlled by the controlunit, the variable load being connected in series with the LED assembly.12. The lighting application according to claim 11 wherein the variableload comprises a switchable resistance.
 13. The lighting applicationaccording to claim 11 wherein a load characteristic of the variable loadis varied during a part of the periodic input voltage of the LED driverwhile the input voltage is monitored.
 14. The lighting applicationaccording to claim 13 wherein the part is a trailing part.
 15. An LEDdriver for powering an LED assembly comprising at least one LED, the LEDdriver comprising a converter for converting a periodic input voltage toa supply current for powering the LED assembly, the converter havinginput terminals for receiving the periodic input voltage, the LED driverfurther comprising an input buffer for providing a current to the inputterminals and a control unit, the LED driver further comprising aswitching element connected between the input terminals and the inputbuffer for opening and closing a current path from the input buffer tothe input terminals and wherein the control unit is further arranged tocontrol the switching element based on an input signal representing theperiodic input voltage, the control unit being configured to close theswitching element in each period of the periodic input voltage during atime when the periodic input voltage is below a minimum level requiredto supply the converter.
 16. The LED driver according to claim 15wherein the input buffer comprises a capacitor or capacitor assembly.17. The LED driver according to claim 15 wherein the switching elementcomprises a first switch for connecting and disconnection a firstterminal of the input buffer to a first terminal of the input terminals.18. The LED driver according to claim 15 wherein the switching elementis further arranged to open and close a current path from the inputbuffer to a supply terminal, in use, connected to the periodic supplyvoltage.
 19. The LED driver according to claim 15 further comprising arectifier for rectifying an AC input voltage to obtain the periodicinput voltage, the rectifier having AC input terminals for receiving theAC input voltage, the switching element comprising a first switch forconnecting a terminal of the input buffer to either a first or a secondterminal of the AC input terminals.
 20. The LED driver according toclaim 19 wherein the switching element comprises a second switch forconnecting a further terminal of the input buffer to an input terminalof the converter.
 21. The LED driver according to claim 15, wherein thecontrol unit is arranged to synchronize an on/off duty-cycle of thesupply current with the periodic input voltage.
 22. The LED driveraccording to claim 15, wherein the control unit is arranged to controlthe LED driver to apply a reduced supply current to the LED assemblywhen the LED assembly is powered from the input buffer.
 23. The LEDdriver according to claim 15, wherein the input buffer comprises acapacitor and wherein the switching element enables the periodic supplyvoltage and a voltage over the capacitor to be added.
 24. The LED driveraccording to claim 15, further comprising a filter capacitor.
 25. TheLED driver according to claim 24 wherein the switching element isarranged to interrupt a current path from the input buffer to the filtercapacitor thereby disabling, in use, the input buffer to discharge tothe filter capacitor.